| Message ID | 1439844063-7957-9-git-send-email-john.stultz@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > On Mon, 17 Aug 2015, John Stultz wrote: > >> From: Shaohua Li <shli@fb.com> >> >> >From time to time we saw TSC is marked as unstable in our systems, while > > Stray '>' > >> the CPUs declare to have stable TSC. Looking at the clocksource unstable >> detection, there are two problems: >> - watchdog clock source wrap. HPET is the most common watchdog clock >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter >> can wrap in about 5 minutes. >> - threshold isn't scaled against interval. The threshold is 0.0625s in >> 0.5s interval. What if the actual interval is bigger than 0.5s? >> >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. >> Heavy network stack softirq can hog a cpu. IPMI driver can disable >> interrupt for a very long time. > > And they hold off the timer softirq for more than a second? Don't you > think that's the problem which needs to be fixed? Though this is an issue I've experienced (and tried unsuccessfully to fix in a more complicated way) with the RT kernel, where high priority tasks blocked the watchdog long enough that we'd disqualify the TSC. Ideally that sort of high-priority RT busyness would be avoided, but its also a pain to have false positive trigger when doing things like stress testing. >> The first problem is mostly we are suffering I think. > > So you think that's the root cause and because your patch makes it go > away it's not necessary to know for sure, right? > >> Here is a simple patch to fix the issues. If the waterdog doesn't run > > waterdog? Allergen-free. :) >> for a long time, we ignore the detection. > > What's 'long time'? Please explain the numbers chosen. > >> This should work for the two > > Emphasis on 'should'? > >> problems. For the second one, we probably doen't need to scale if the >> interval isn't very long. > > -ENOPARSE > >> @@ -122,9 +122,10 @@ static int clocksource_watchdog_kthread(void *data); >> static void __clocksource_change_rating(struct clocksource *cs, int rating); >> >> /* >> - * Interval: 0.5sec Threshold: 0.0625s >> + * Interval: 0.5sec MaxInterval: 1s Threshold: 0.0625s >> */ >> #define WATCHDOG_INTERVAL (HZ >> 1) >> +#define WATCHDOG_MAX_INTERVAL_NS (NSEC_PER_SEC) >> #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) >> >> static void clocksource_watchdog_work(struct work_struct *work) >> @@ -217,7 +218,9 @@ static void clocksource_watchdog(unsigned long data) >> continue; >> >> /* Check the deviation from the watchdog clocksource. */ >> - if ((abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD)) { >> + if ((abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) && >> + cs_nsec < WATCHDOG_MAX_INTERVAL_NS && >> + wd_nsec < WATCHDOG_MAX_INTERVAL_NS) { > > So that adds a new opportunity for undiscovered wreckage: > > clocksource_watchdog(); > .... <--- SMI skews TSC > looong_irq_disabled_region(); > .... > clocksource_watchdog(); <--- Does not detect skew > > and it will not detect it later on if that SMI was a one time event. > > So 'fixing' the watchdog is the wrong approach. Fixing the stuff which > prevents the watchdog to run is the proper thing to do. I'm not sure here. I feel like these delay-caused false positives (I've seen similar reports w/ VMs being stalled) are more common then one-off SMI TSC skews. There are hard lines in the timekeeping code, where we do say: Don't delay us past X or we can't really handle it, but in this case, the main clocksource is fine and the limit is being caused by the watchdog. So I think some sort of a solution to remove this restriction would be good. We don't want to needlessly punish fine hardware because our checks for bad hardware add extra restrictions. That said, I agree the "should"s and other vague qualifiers in the commit description you point out should have more specifics to back things up. And I'm fine delaying this (and the follow-on) patch until those details are provided. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Mon, Aug 17, 2015 at 7:57 PM, Shaohua Li <shli@fb.com> wrote: > On Mon, Aug 17, 2015 at 03:17:28PM -0700, John Stultz wrote: >> That said, I agree the "should"s and other vague qualifiers in the >> commit description you point out should have more specifics to back >> things up. And I'm fine delaying this (and the follow-on) patch until >> those details are provided. > > It's not something I guess. We do see the issue from time to time. The > IPMI driver accesses some IO ports in softirq and hog cpu for a very > long time, then the watchdog alert. The false alert on the other hand > has very worse effect. It forces to use HPET as clocksource, which has > very big performance penality. We can't even manually switch back to TSC > as current interface doesn't allow us to do it, then we can only reboot > the system. I agree the driver should be fixed, but the watchdog has > false alert, we definitively should fix it. I think Thomas is requesting that some of the vague terms be quantified. Seeing the issue "from time to time" isn't super informative. When the IPMI driver hogs the cpu "for a very long time", how long does that actually take? You've provided the HPET frequency, and the wrapping interval on your hardware. Do these intervals all line up properly? I sympathize that the "show-your-work" math problem aspect of this request might be a little remedial and irritating, esp when the patch fixes the problem for you. But its important, so later on when some bug crops up in near by code, folks can easily repeat your calculation and know the problem isn't from your code. > The 1s interval is arbitary. If you think there is better way to fix the > issue, please let me know. I don't think 1s is necessarily arbitrary. Maybe not much conscious thought was put into it, but clearly .001 sec wasn't chosen, nor 10minutes for a reason. So given the intervals you're seeing the problem with, would maybe a larger max interval (say 30-seconds) make more or less sense? What would the tradeoffs be? (ie: Would that exclude clocksources with faster wraps from being used as watchdogs, with your patches?). I'm sure an good interval could be chosen with some thought, and the rational be explained. :) thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Tue, Aug 18, 2015 at 1:38 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > On Mon, 17 Aug 2015, John Stultz wrote: >> On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@linutronix.de> wrote: >> > On Mon, 17 Aug 2015, John Stultz wrote: >> > >> >> From: Shaohua Li <shli@fb.com> >> >> >> >> >From time to time we saw TSC is marked as unstable in our systems, while >> > >> > Stray '>' >> > >> >> the CPUs declare to have stable TSC. Looking at the clocksource unstable >> >> detection, there are two problems: >> >> - watchdog clock source wrap. HPET is the most common watchdog clock >> >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter >> >> can wrap in about 5 minutes. >> >> - threshold isn't scaled against interval. The threshold is 0.0625s in >> >> 0.5s interval. What if the actual interval is bigger than 0.5s? >> >> >> >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. >> >> Heavy network stack softirq can hog a cpu. IPMI driver can disable >> >> interrupt for a very long time. >> > >> > And they hold off the timer softirq for more than a second? Don't you >> > think that's the problem which needs to be fixed? >> >> Though this is an issue I've experienced (and tried unsuccessfully to >> fix in a more complicated way) with the RT kernel, where high priority >> tasks blocked the watchdog long enough that we'd disqualify the TSC. > > Did it disqualify the watchdog due to HPET wraparounds (5 minutes) or > due to the fixed threshold being applied? This was years ago, but in my experience, the watchdog false positives were due to HPET wraparounds. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Tue, Aug 18, 2015 at 12:28 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > On Tue, 18 Aug 2015, John Stultz wrote: >> On Tue, Aug 18, 2015 at 1:38 AM, Thomas Gleixner <tglx@linutronix.de> wrote: >> > On Mon, 17 Aug 2015, John Stultz wrote: >> >> On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@linutronix.de> wrote: >> >> > On Mon, 17 Aug 2015, John Stultz wrote: >> >> > >> >> >> From: Shaohua Li <shli@fb.com> >> >> >> >> >> >> >From time to time we saw TSC is marked as unstable in our systems, while >> >> > >> >> > Stray '>' >> >> > >> >> >> the CPUs declare to have stable TSC. Looking at the clocksource unstable >> >> >> detection, there are two problems: >> >> >> - watchdog clock source wrap. HPET is the most common watchdog clock >> >> >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter >> >> >> can wrap in about 5 minutes. >> >> >> - threshold isn't scaled against interval. The threshold is 0.0625s in >> >> >> 0.5s interval. What if the actual interval is bigger than 0.5s? >> >> >> >> >> >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. >> >> >> Heavy network stack softirq can hog a cpu. IPMI driver can disable >> >> >> interrupt for a very long time. >> >> > >> >> > And they hold off the timer softirq for more than a second? Don't you >> >> > think that's the problem which needs to be fixed? >> >> >> >> Though this is an issue I've experienced (and tried unsuccessfully to >> >> fix in a more complicated way) with the RT kernel, where high priority >> >> tasks blocked the watchdog long enough that we'd disqualify the TSC. >> > >> > Did it disqualify the watchdog due to HPET wraparounds (5 minutes) or >> > due to the fixed threshold being applied? >> >> This was years ago, but in my experience, the watchdog false positives >> were due to HPET wraparounds. > > Blocking stuff for 5 minutes is insane .... Yea. It was usually due to -RT stress testing, which keept the machines busy for quite awhile. But again, if you have machines being maxed out with networking load, etc, even for long amounts of time, we still want to avoid false positives. Because after the watchdog disqualifies the TSC, the only clocksources left wrap around much sooner, and we're more likely to then actually lose time during the next load spike. Cc'ing Clark and Steven to see if its something they still run into, and maybe they can help validate the patch. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Wed, Aug 26, 2015 at 10:15:33AM -0700, Shaohua Li wrote: > On Tue, Aug 18, 2015 at 10:18:09PM +0200, Thomas Gleixner wrote: > > On Tue, 18 Aug 2015, John Stultz wrote: > > > On Tue, Aug 18, 2015 at 12:28 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > > > > On Tue, 18 Aug 2015, John Stultz wrote: > > > >> On Tue, Aug 18, 2015 at 1:38 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > > >> > On Mon, 17 Aug 2015, John Stultz wrote: > > > >> >> On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > > > >> >> > On Mon, 17 Aug 2015, John Stultz wrote: > > > >> >> > > > > >> >> >> From: Shaohua Li <shli@fb.com> > > > >> >> >> > > > >> >> >> >From time to time we saw TSC is marked as unstable in our systems, while > > > >> >> > > > > >> >> > Stray '>' > > > >> >> > > > > >> >> >> the CPUs declare to have stable TSC. Looking at the clocksource unstable > > > >> >> >> detection, there are two problems: > > > >> >> >> - watchdog clock source wrap. HPET is the most common watchdog clock > > > >> >> >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter > > > >> >> >> can wrap in about 5 minutes. > > > >> >> >> - threshold isn't scaled against interval. The threshold is 0.0625s in > > > >> >> >> 0.5s interval. What if the actual interval is bigger than 0.5s? > > > >> >> >> > > > >> >> >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. > > > >> >> >> Heavy network stack softirq can hog a cpu. IPMI driver can disable > > > >> >> >> interrupt for a very long time. > > > >> >> > > > > >> >> > And they hold off the timer softirq for more than a second? Don't you > > > >> >> > think that's the problem which needs to be fixed? > > > >> >> > > > >> >> Though this is an issue I've experienced (and tried unsuccessfully to > > > >> >> fix in a more complicated way) with the RT kernel, where high priority > > > >> >> tasks blocked the watchdog long enough that we'd disqualify the TSC. > > > >> > > > > >> > Did it disqualify the watchdog due to HPET wraparounds (5 minutes) or > > > >> > due to the fixed threshold being applied? > > > >> > > > >> This was years ago, but in my experience, the watchdog false positives > > > >> were due to HPET wraparounds. > > > > > > > > Blocking stuff for 5 minutes is insane .... > > > > > > Yea. It was usually due to -RT stress testing, which keept the > > > machines busy for quite awhile. But again, if you have machines being > > > maxed out with networking load, etc, even for long amounts of time, we > > > still want to avoid false positives. Because after the watchdog > > > > The networking softirq does not hog the other softirqs. It has a limit > > on processing loops and then goes back to let the other softirqs be > > handled. So no, I doubt that heavy networking can cause this. If it > > does then we have some other way more serious problems. > > > > I can see the issue with RT stress testing, but not with networking in > > mainline. > > Ok, the issue is triggerd in my kvm guest, I guess it's easier to > trigger in kvm because hpet is 100Mhz. > > [ 135.930067] clocksource: timekeeping watchdog: Marking clocksource 'tsc' as unstable because the skew is too large: > [ 135.930095] clocksource: 'hpet' wd_now: 2bc19ea0 wd_last: 6c4e5570 mask: ffffffff > [ 135.930105] clocksource: 'tsc' cs_now: 481250b45b cs_last: 219e6efb50 mask: ffffffffffffffff > [ 135.938750] clocksource: Switched to clocksource hpet > > The HPET clock is 100MHz, CPU speed is 2200MHz, kvm is passed correct cpu > info, so guest cpuinfo shows TSC is stable. > > hpet interval is ((0x2bc19ea0 - 0x6c4e5570) & 0xffffffff) / 100000000 = 32.1s. > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > 32.1 + 42.9 = 75 > > The example shows hpet wraps, while tsc is marked unstable Thomas & John, Is this data enough to prove TSC unstable issue can be triggered by HPET wrap? I can resend the patch with the data included. Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Mon, 31 Aug 2015, Shaohua Li wrote: > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > > > 32.1 + 42.9 = 75 > > > > The example shows hpet wraps, while tsc is marked unstable > > Thomas & John, > Is this data enough to prove TSC unstable issue can be triggered by HPET > wrap? I can resend the patch with the data included. Well, it's enough data to prove: - that keeping a VM off the CPU for 75 seconds is insane. - that emulating the HPET with 100MHz shortens the HPET wraparound by a factor of 7 compared to real hardware. With a realist HPET frequency you have about 300 seconds. Who though that using 100MHz HPET frequency is a brilliant idea? So we should add crappy heuristics to the watchdog just to workaround virt insanities? I'm not convinced. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Mon, Aug 31, 2015 at 11:47:52PM +0200, Thomas Gleixner wrote: > On Mon, 31 Aug 2015, Shaohua Li wrote: > > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > > > > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > > > > > 32.1 + 42.9 = 75 > > > > > > The example shows hpet wraps, while tsc is marked unstable > > > > Thomas & John, > > Is this data enough to prove TSC unstable issue can be triggered by HPET > > wrap? I can resend the patch with the data included. > > Well, it's enough data to prove: > > - that keeping a VM off the CPU for 75 seconds is insane. It wraps in 42.9s. 42.9s isn't a long time hard to block. I don’t think it's just VM off. A softirq can hog the cpu. > - that emulating the HPET with 100MHz shortens the HPET wraparound by > a factor of 7 compared to real hardware. With a realist HPET > frequency you have about 300 seconds. > > Who though that using 100MHz HPET frequency is a brilliant idea? I'm not a VM expert. My guess is the 100Mhz can reduce interrupt. It’s insane hypervisor updates HPET count in 14.3Mhz. Switching to HPET can introduce even higher overhead in virtual, because of the vmexit of iomemory access > So we should add crappy heuristics to the watchdog just to workaround > virt insanities? I'm not convinced. This is a real issue which could impact performance seriously. Though the data is collected in vm, we do see the issue happens in physical machines too. The watchdog clock source shows restriction here apparently, it deserves an improvement if we can do. I'm happy to hear from you if there is better solution, but we shouldn't pretend there is no issue here. Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Mon, 31 Aug 2015, Shaohua Li wrote: > On Mon, Aug 31, 2015 at 11:47:52PM +0200, Thomas Gleixner wrote: > > On Mon, 31 Aug 2015, Shaohua Li wrote: > > > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > > > > > > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > > > > > > > 32.1 + 42.9 = 75 > > > > > > > > The example shows hpet wraps, while tsc is marked unstable > > > > > > Thomas & John, > > > Is this data enough to prove TSC unstable issue can be triggered by HPET > > > wrap? I can resend the patch with the data included. > > > > Well, it's enough data to prove: > > > > - that keeping a VM off the CPU for 75 seconds is insane. > > It wraps in 42.9s. 42.9s isn't a long time hard to block. I don’t think You think that blocking softirq execution for 42.9 seconds is normal? Seems we are living in a different universe. > it's just VM off. A softirq can hog the cpu. I still want to see prove of that. There is just handwaving about that, but nobody has provided proper data to back that up. > > - that emulating the HPET with 100MHz shortens the HPET wraparound by > > a factor of 7 compared to real hardware. With a realist HPET > > frequency you have about 300 seconds. > > > > Who though that using 100MHz HPET frequency is a brilliant idea? > > I'm not a VM expert. My guess is the 100Mhz can reduce interrupt. It’s > insane hypervisor updates HPET count in 14.3Mhz. Switching to HPET can > introduce even higher overhead in virtual, because of the vmexit of > iomemory access Sorry, that does not make any sense at all. - How does 100Mhz HPET frequency reduce interrupts? - What's insane about a lower emulated HPET frequency? - We all know that switching to HPET is more expensive than just using TSC. That's not the question at all and completely unrelated to the 100MHz HPET emulation frequency. > > So we should add crappy heuristics to the watchdog just to workaround > > virt insanities? I'm not convinced. > > This is a real issue which could impact performance seriously. Though > the data is collected in vm, we do see the issue happens in physical > machines too. And what's the exact reason for this on physical machines? Some magic softirq hog again for which you cannot provide proof? > The watchdog clock source shows restriction here apparently, it > deserves an improvement if we can do. The restriction in a sane environment is 300 seconds. And the only fallout on physical hardware which we have seen so far is on preempt-RT where the softirq can actually be blocked by RT hogs, but that's a completely different issue and has nothing to do with the situation in mainline. > I'm happy to hear from you if there is better solution, but we > shouldn't pretend there is no issue here. I'm not pretending anything. I'm merily refusing to accept that change w/o a proper explanation WHY the watchdog fails on physical hardware, i.e. WHY it does not run for more than 300 seconds. Thanks, tglx
On Tue, Sep 01, 2015 at 07:13:40PM +0200, Thomas Gleixner wrote: > On Mon, 31 Aug 2015, Shaohua Li wrote: > > On Mon, Aug 31, 2015 at 11:47:52PM +0200, Thomas Gleixner wrote: > > > On Mon, 31 Aug 2015, Shaohua Li wrote: > > > > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > > > > > > > > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > > > > > > > > > 32.1 + 42.9 = 75 > > > > > > > > > > The example shows hpet wraps, while tsc is marked unstable > > > > > > > > Thomas & John, > > > > Is this data enough to prove TSC unstable issue can be triggered by HPET > > > > wrap? I can resend the patch with the data included. > > > > > > Well, it's enough data to prove: > > > > > > - that keeping a VM off the CPU for 75 seconds is insane. > > > > It wraps in 42.9s. 42.9s isn't a long time hard to block. I don’t think > > You think that blocking softirq execution for 42.9 seconds is normal? > Seems we are living in a different universe. I don't say it's normal. I say it's not hard to trigger. > > it's just VM off. A softirq can hog the cpu. > > I still want to see prove of that. There is just handwaving about > that, but nobody has provided proper data to back that up. I showed you the TSC runs 75s, while hpet wraps. What info you think can prove this? > > > - that emulating the HPET with 100MHz shortens the HPET wraparound by > > > a factor of 7 compared to real hardware. With a realist HPET > > > frequency you have about 300 seconds. > > > > > > Who though that using 100MHz HPET frequency is a brilliant idea? > > > > I'm not a VM expert. My guess is the 100Mhz can reduce interrupt. It’s > > insane hypervisor updates HPET count in 14.3Mhz. Switching to HPET can > > introduce even higher overhead in virtual, because of the vmexit of > > iomemory access > > Sorry, that does not make any sense at all. > > - How does 100Mhz HPET frequency reduce interrupts? > > - What's insane about a lower emulated HPET frequency? > > - We all know that switching to HPET is more expensive than just > using TSC. That's not the question at all and completely > unrelated to the 100MHz HPET emulation frequency. It's meaningless to argue about HPET frequency. The code should not just work for 14.3Mhz HPET. > > > So we should add crappy heuristics to the watchdog just to workaround > > > virt insanities? I'm not convinced. > > > > This is a real issue which could impact performance seriously. Though > > the data is collected in vm, we do see the issue happens in physical > > machines too. > > And what's the exact reason for this on physical machines? Some magic > softirq hog again for which you cannot provide proof? > > > The watchdog clock source shows restriction here apparently, it > > deserves an improvement if we can do. > > The restriction in a sane environment is 300 seconds. And the only > fallout on physical hardware which we have seen so far is on > preempt-RT where the softirq can actually be blocked by RT hogs, but > that's a completely different issue and has nothing to do with the > situation in mainline. > > > I'm happy to hear from you if there is better solution, but we > > shouldn't pretend there is no issue here. > > I'm not pretending anything. I'm merily refusing to accept that change > w/o a proper explanation WHY the watchdog fails on physical hardware, > i.e. WHY it does not run for more than 300 seconds. It's meaningless to argue about virtual/physical machine too. Linux works for both virtual/physical machines. What about acpi_pm clocksource then? It wraps in abour 5s. It's sane HPET is disabled and acpi_pm is used for watchdog. Do you still think 5s is long? Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Tue, 1 Sep 2015, Shaohua Li wrote: > On Tue, Sep 01, 2015 at 07:13:40PM +0200, Thomas Gleixner wrote: > > On Mon, 31 Aug 2015, Shaohua Li wrote: > > > On Mon, Aug 31, 2015 at 11:47:52PM +0200, Thomas Gleixner wrote: > > > > On Mon, 31 Aug 2015, Shaohua Li wrote: > > > > > > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s > > > > > > > > > > > > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s > > > > > > > > > > > > 32.1 + 42.9 = 75 > > > > > > > > > > > > The example shows hpet wraps, while tsc is marked unstable > > > > > > > > > > Thomas & John, > > > > > Is this data enough to prove TSC unstable issue can be triggered by HPET > > > > > wrap? I can resend the patch with the data included. > > > > > > > > Well, it's enough data to prove: > > > > > > > > - that keeping a VM off the CPU for 75 seconds is insane. > > > > > > It wraps in 42.9s. 42.9s isn't a long time hard to block. I don’t think > > > > You think that blocking softirq execution for 42.9 seconds is normal? > > Seems we are living in a different universe. > > I don't say it's normal. I say it's not hard to trigger. So and because its not hard to trigger, we cure the symptom and do not think about the insanity of blocking the watchdog for 42+ or 300+ seconds. > > > it's just VM off. A softirq can hog the cpu. > > > > I still want to see prove of that. There is just handwaving about > > that, but nobody has provided proper data to back that up. > > I showed you the TSC runs 75s, while hpet wraps. What info you think can > prove this? You prove nothing. You showed me the symptom, but you never showed real data that a softirq hogs the cpu for 300+ seconds. Still you keep claiming that. You did neither provide a proper explanation WHY your VM test blocked the watchdog for 75 seconds. No, you merily showed me the numbers. And just because the numbers explain the symptom, that's no justification WHY we should cure the symptom instead of looking at the root cause. > > > > - that emulating the HPET with 100MHz shortens the HPET wraparound by > > > > a factor of 7 compared to real hardware. With a realist HPET > > > > frequency you have about 300 seconds. > > > > > > > > Who though that using 100MHz HPET frequency is a brilliant idea? > > > > > > I'm not a VM expert. My guess is the 100Mhz can reduce interrupt. It’s > > > insane hypervisor updates HPET count in 14.3Mhz. Switching to HPET can > > > introduce even higher overhead in virtual, because of the vmexit of > > > iomemory access > > > > Sorry, that does not make any sense at all. > > > > - How does 100Mhz HPET frequency reduce interrupts? > > > > - What's insane about a lower emulated HPET frequency? > > > > - We all know that switching to HPET is more expensive than just > > using TSC. That's not the question at all and completely > > unrelated to the 100MHz HPET emulation frequency. > > It's meaningless to argue about HPET frequency. The code should not just > work for 14.3Mhz HPET. You carefully avoid to answer any of my questions, but you expect from me to accept your wild guess argumentations? > > I'm not pretending anything. I'm merily refusing to accept that change > > w/o a proper explanation WHY the watchdog fails on physical hardware, > > i.e. WHY it does not run for more than 300 seconds. > > It's meaningless to argue about virtual/physical machine too. Linux > works for both virtual/physical machines. That has nothing to do with virt vs. physical. Virtualization is meant to provide proper hardware emulation. Does Linux work with a buggy APIC emulation? Not at all, but you expect that it just handles an insane HPET emulation, right? > What about acpi_pm clocksource then? It wraps in abour 5s. It's sane > HPET is disabled and acpi_pm is used for watchdog. Do you still think 5s > is long? Yes, five seconds is long. It's more than 10 billions worth of cpu cycles on a 2GHz machine. If your desktop stalls for 5 seconds you are probably less enthusiatic. Again, I'm not against making the watchdog more robust, but I'm against curing the symptoms. As long as you refuse to provide proper explanations WHY the watchdog is blocked unduly long, this is going nowhere. Thanks, tglx
On Tue, 1 Sep 2015 11:14:17 -0700 Shaohua Li <shli@fb.com> wrote: > > You think that blocking softirq execution for 42.9 seconds is normal? > > Seems we are living in a different universe. > > I don't say it's normal. I say it's not hard to trigger. > > > > it's just VM off. A softirq can hog the cpu. > > Please provide a test case that shows the softirq hogging the cpu for over 40 seconds. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Tue, Sep 01, 2015 at 03:35:18PM -0400, Steven Rostedt wrote: > On Tue, 1 Sep 2015 11:14:17 -0700 > Shaohua Li <shli@fb.com> wrote: > > > > You think that blocking softirq execution for 42.9 seconds is normal? > > > Seems we are living in a different universe. > > > > I don't say it's normal. I say it's not hard to trigger. > > > > > > it's just VM off. A softirq can hog the cpu. > > > > > Please provide a test case that shows the softirq hogging the cpu for > over 40 seconds. Do not overlook the MAX_SOFTIRQ_* logic while you're there trying to explain this ;-) A softirq running significantly longer than 2 jiffies and not falling back to ksoftirq is a plain bug. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 841b72f..8417c83 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -122,9 +122,10 @@ static int clocksource_watchdog_kthread(void *data); static void __clocksource_change_rating(struct clocksource *cs, int rating); /* - * Interval: 0.5sec Threshold: 0.0625s + * Interval: 0.5sec MaxInterval: 1s Threshold: 0.0625s */ #define WATCHDOG_INTERVAL (HZ >> 1) +#define WATCHDOG_MAX_INTERVAL_NS (NSEC_PER_SEC) #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) static void clocksource_watchdog_work(struct work_struct *work) @@ -217,7 +218,9 @@ static void clocksource_watchdog(unsigned long data) continue; /* Check the deviation from the watchdog clocksource. */ - if ((abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD)) { + if ((abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) && + cs_nsec < WATCHDOG_MAX_INTERVAL_NS && + wd_nsec < WATCHDOG_MAX_INTERVAL_NS) { pr_warn("timekeeping watchdog: Marking clocksource '%s' as unstable because the skew is too large:\n", cs->name); pr_warn(" '%s' wd_now: %llx wd_last: %llx mask: %llx\n",