| Message ID | 20231208002342.367117-8-qyousef@layalina.io |
|---|---|
| State | New |
| Headers | show |
| Series | sched: cpufreq: Remove magic hardcoded numbers from margins | expand |
On Sun, Dec 10, 2023 at 9:40 PM Qais Yousef <qyousef@layalina.io> wrote: > > On 12/08/23 19:06, Rafael J. Wysocki wrote: > > On Fri, Dec 8, 2023 at 1:24 AM Qais Yousef <qyousef@layalina.io> wrote: > > > > > > The new tunable, response_time_ms, allow us to speed up or slow down > > > the response time of the policy to meet the perf, power and thermal > > > characteristic desired by the user/sysadmin. There's no single universal > > > trade-off that we can apply for all systems even if they use the same > > > SoC. The form factor of the system, the dominant use case, and in case > > > of battery powered systems, the size of the battery and presence or > > > absence of active cooling can play a big role on what would be best to > > > use. > > > > > > The new tunable provides sensible defaults, but yet gives the power to > > > control the response time to the user/sysadmin, if they wish to. > > > > > > This tunable is applied before we apply the DVFS headroom. > > > > > > The default behavior of applying 1.25 headroom can be re-instated easily > > > now. But we continue to keep the min required headroom to overcome > > > hardware limitation in its speed to change DVFS. And any additional > > > headroom to speed things up must be applied by userspace to match their > > > expectation for best perf/watt as it dictates a type of policy that will > > > be better for some systems, but worse for others. > > > > > > There's a whitespace clean up included in sugov_start(). > > > > > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> > > > > I thought that there was an agreement to avoid adding any new tunables > > to schedutil. > > Oh. I didn't know that. > > What alternatives do we have? I couldn't see how can we universally make the > response work for every possible system (not just SoC, but different platforms > with same SoC even) and workloads. We see big power saving with no or little > perf impact on many workloads when not applying the current 125%. Others want > to push it faster under gaming scenarios etc to get more stable FPS. > > Hopefully uclamp will make the need for this tuning obsolete over time. But > until userspace gains critical mass; I can't see how we can know best > trade-offs for all myriads of use cases/systems. > > Some are happy to gain more perf and lose power. Others prefer to save power > over perf. DVFS response time plays a critical role in this trade-off and I'm > not sure how we can crystal ball it without delegating. I understand the motivation, but counter-arguments are based on the experience with the cpufreq governors predating schedutil, especially ondemand. Namely, at one point people focused on adjusting all of the governor tunables to their needs without contributing any code or even insights back, so when schedutil was introduced, a decision was made to reduce the tunability to a minimum (preferably no tunables at all, but it turned out to be hard to avoid the one tunable existing today). Peter was involved in those discussions and I think that the point made then is still valid. The headroom formula was based on the observation that it would be a good idea to have some headroom in the majority of cases and on the balance between the simplicity of computation and general suitability. Of course, it is hard to devise a single value that will work for everyone, but tunables complicate things from the maintenance perspective. For example, the more tunables there are, the harder it is to make changes without altering the behavior in ways that will break someone's setup.
On 12/11/23 21:20, Rafael J. Wysocki wrote: > I understand the motivation, but counter-arguments are based on the > experience with the cpufreq governors predating schedutil, especially > ondemand. Namely, at one point people focused on adjusting all of the > governor tunables to their needs without contributing any code or even > insights back, so when schedutil was introduced, a decision was made > to reduce the tunability to a minimum (preferably no tunables at all, > but it turned out to be hard to avoid the one tunable existing today). > Peter was involved in those discussions and I think that the point > made then is still valid. > > The headroom formula was based on the observation that it would be a > good idea to have some headroom in the majority of cases and on the > balance between the simplicity of computation and general suitability. > > Of course, it is hard to devise a single value that will work for > everyone, but tunables complicate things from the maintenance > perspective. For example, the more tunables there are, the harder it > is to make changes without altering the behavior in ways that will > break someone's setup. Okay thanks for the insights Rafael! I hope the matter is open for debate at least. I do agree and share the sentiment and if there's another way to avoid the tunable I'm all for going to try it out. I just personally failed to see how can we do this without delegating. And the current choice of 25% headroom is too aggressive for modern hardware IMHO. I'm not sure we can pick a value that will truly work for most use cases. In mobile world, it is really hard to cover all use cases. Different OEMs tend to focus on different use cases and design their systems to optimally work for those. And those use cases don't necessarily hit the same bottlenecks on different systems. If we consider all the possible systems that Linux gets incorporated in, it is even harder to tell what's a sensible default. And generally if there's indeed a default that works for most users, what should we do if we fall into the minority where this default is not suitable for us? I think we need to handle this still. So we need a way somehow even if this proposal doesn't hit the mark. Although again, I hope the matter is open for debate. The only ultimate solution I see is userspace becoming fully uclamp aware and tell us their perf requirements. Then this value will be NOP as we have direct info from the use cases to help us give them the performance they need when they need it. And if their usage ends up with bad perf or power, we can at least shift the blame for their bad usage :-) /me runs But this is years from hitting the critical mass. We need to get to a point where we can enable uclamp config by default as not all distros enable it still. Anyway, looking forward to learning more on how we can do better. Thanks! -- Qais Yousef
On 12/08/23 00:23, Qais Yousef wrote: > +static inline u64 sugov_calc_freq_response_ms(struct sugov_policy *sg_policy) > +{ > + int cpu = cpumask_first(sg_policy->policy->cpus); > + unsigned long cap = arch_scale_cpu_capacity(cpu); > + unsigned int max_freq, sec_max_freq; > + > + max_freq = sg_policy->policy->cpuinfo.max_freq; > + sec_max_freq = __resolve_freq(sg_policy->policy, > + max_freq - 1, > + CPUFREQ_RELATION_H); > + > + /* > + * We will request max_freq as soon as util crosses the capacity at > + * second highest frequency. So effectively our response time is the > + * util at which we cross the cap@2nd_highest_freq. > + */ > + cap = sec_max_freq * cap / max_freq; > + > + return approximate_runtime(cap + 1); After Linus problem (and more testing) I realize this is not correct. This value is correct for the biggest core only, for smaller cores I must stretch time with capacity. I have similar invariance issues that I need to address in this series and uclamp max aggregation series.
diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst index 6adb7988e0eb..fa0d602a920e 100644 --- a/Documentation/admin-guide/pm/cpufreq.rst +++ b/Documentation/admin-guide/pm/cpufreq.rst @@ -417,7 +417,7 @@ is passed by the scheduler to the governor callback which causes the frequency to go up to the allowed maximum immediately and then draw back to the value returned by the above formula over time. -This governor exposes only one tunable: +This governor exposes two tunables: ``rate_limit_us`` Minimum time (in microseconds) that has to pass between two consecutive @@ -427,6 +427,21 @@ This governor exposes only one tunable: The purpose of this tunable is to reduce the scheduler context overhead of the governor which might be excessive without it. +``respone_time_ms`` + Amount of time (in milliseconds) required to ramp the policy from + lowest to highest frequency. Can be decreased to speed up the + responsiveness of the system, or increased to slow the system down in + hope to save power. The best perf/watt will depend on the system + characteristics and the dominant workload you expect to run. For + userspace that has smart context on the type of workload running (like + in Android), one can tune this to suite the demand of that workload. + + Note that when slowing the response down, you can end up effectively + chopping off the top frequencies for that policy as the util is capped + to 1024. On HMP systems this chopping effect will only occur on the + biggest core whose capacity is 1024. Don't rely on this behavior as + this is a limitation that can hopefully be improved in the future. + This governor generally is regarded as a replacement for the older `ondemand`_ and `conservative`_ governors (described below), as it is simpler and more tightly integrated with the CPU scheduler, its overhead in terms of CPU context diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 9875284ca6e4..15c397ce3252 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -533,8 +533,8 @@ void cpufreq_disable_fast_switch(struct cpufreq_policy *policy) } EXPORT_SYMBOL_GPL(cpufreq_disable_fast_switch); -static unsigned int __resolve_freq(struct cpufreq_policy *policy, - unsigned int target_freq, unsigned int relation) +unsigned int __resolve_freq(struct cpufreq_policy *policy, + unsigned int target_freq, unsigned int relation) { unsigned int idx; diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h index 1c5ca92a0555..29c3723653a3 100644 --- a/include/linux/cpufreq.h +++ b/include/linux/cpufreq.h @@ -613,6 +613,9 @@ int cpufreq_driver_target(struct cpufreq_policy *policy, int __cpufreq_driver_target(struct cpufreq_policy *policy, unsigned int target_freq, unsigned int relation); +unsigned int __resolve_freq(struct cpufreq_policy *policy, + unsigned int target_freq, + unsigned int relation); unsigned int cpufreq_driver_resolve_freq(struct cpufreq_policy *policy, unsigned int target_freq); unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy); diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c index 1d4d6025c15f..788208becc13 100644 --- a/kernel/sched/cpufreq_schedutil.c +++ b/kernel/sched/cpufreq_schedutil.c @@ -8,9 +8,12 @@ #define IOWAIT_BOOST_MIN (SCHED_CAPACITY_SCALE / 8) +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, response_time_mult); + struct sugov_tunables { struct gov_attr_set attr_set; unsigned int rate_limit_us; + unsigned int response_time_ms; }; struct sugov_policy { @@ -22,6 +25,7 @@ struct sugov_policy { raw_spinlock_t update_lock; u64 last_freq_update_time; s64 freq_update_delay_ns; + unsigned int freq_response_time_ms; unsigned int next_freq; unsigned int cached_raw_freq; @@ -59,6 +63,70 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu); /************************ Governor internals ***********************/ +static inline u64 sugov_calc_freq_response_ms(struct sugov_policy *sg_policy) +{ + int cpu = cpumask_first(sg_policy->policy->cpus); + unsigned long cap = arch_scale_cpu_capacity(cpu); + unsigned int max_freq, sec_max_freq; + + max_freq = sg_policy->policy->cpuinfo.max_freq; + sec_max_freq = __resolve_freq(sg_policy->policy, + max_freq - 1, + CPUFREQ_RELATION_H); + + /* + * We will request max_freq as soon as util crosses the capacity at + * second highest frequency. So effectively our response time is the + * util at which we cross the cap@2nd_highest_freq. + */ + cap = sec_max_freq * cap / max_freq; + + return approximate_runtime(cap + 1); +} + +static inline void sugov_update_response_time_mult(struct sugov_policy *sg_policy) +{ + unsigned long mult; + int cpu; + + if (unlikely(!sg_policy->freq_response_time_ms)) + sg_policy->freq_response_time_ms = sugov_calc_freq_response_ms(sg_policy); + + mult = sg_policy->freq_response_time_ms * SCHED_CAPACITY_SCALE; + mult /= sg_policy->tunables->response_time_ms; + + if (SCHED_WARN_ON(!mult)) + mult = SCHED_CAPACITY_SCALE; + + for_each_cpu(cpu, sg_policy->policy->cpus) + per_cpu(response_time_mult, cpu) = mult; +} + +/* + * Shrink or expand how long it takes to reach the maximum performance of the + * policy. + * + * sg_policy->freq_response_time_ms is a constant value defined by PELT + * HALFLIFE and the capacity of the policy (assuming HMP systems). + * + * sg_policy->tunables->response_time_ms is a user defined response time. By + * setting it lower than sg_policy->freq_response_time_ms, the system will + * respond faster to changes in util, which will result in reaching maximum + * performance point quicker. By setting it higher, it'll slow down the amount + * of time required to reach the maximum OPP. + * + * This should be applied when selecting the frequency. + */ +static inline unsigned long +sugov_apply_response_time(unsigned long util, int cpu) +{ + unsigned long mult; + + mult = per_cpu(response_time_mult, cpu) * util; + + return mult >> SCHED_CAPACITY_SHIFT; +} + static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) { s64 delta_ns; @@ -156,7 +224,10 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual, unsigned long min, unsigned long max) { - /* Add dvfs headroom to actual utilization */ + /* + * Speed up/slow down response timee first then apply DVFS headroom. + */ + actual = sugov_apply_response_time(actual, cpu); actual = apply_dvfs_headroom(actual, cpu); /* Actually we don't need to target the max performance */ if (actual < max) @@ -555,8 +626,42 @@ rate_limit_us_store(struct gov_attr_set *attr_set, const char *buf, size_t count static struct governor_attr rate_limit_us = __ATTR_RW(rate_limit_us); +static ssize_t response_time_ms_show(struct gov_attr_set *attr_set, char *buf) +{ + struct sugov_tunables *tunables = to_sugov_tunables(attr_set); + + return sprintf(buf, "%u\n", tunables->response_time_ms); +} + +static ssize_t +response_time_ms_store(struct gov_attr_set *attr_set, const char *buf, size_t count) +{ + struct sugov_tunables *tunables = to_sugov_tunables(attr_set); + struct sugov_policy *sg_policy; + unsigned int response_time_ms; + + if (kstrtouint(buf, 10, &response_time_ms)) + return -EINVAL; + + /* XXX need special handling for high values? */ + + tunables->response_time_ms = response_time_ms; + + list_for_each_entry(sg_policy, &attr_set->policy_list, tunables_hook) { + if (sg_policy->tunables == tunables) { + sugov_update_response_time_mult(sg_policy); + break; + } + } + + return count; +} + +static struct governor_attr response_time_ms = __ATTR_RW(response_time_ms); + static struct attribute *sugov_attrs[] = { &rate_limit_us.attr, + &response_time_ms.attr, NULL }; ATTRIBUTE_GROUPS(sugov); @@ -744,11 +849,13 @@ static int sugov_init(struct cpufreq_policy *policy) goto stop_kthread; } - tunables->rate_limit_us = cpufreq_policy_transition_delay_us(policy); - policy->governor_data = sg_policy; sg_policy->tunables = tunables; + tunables->rate_limit_us = cpufreq_policy_transition_delay_us(policy); + tunables->response_time_ms = sugov_calc_freq_response_ms(sg_policy); + sugov_update_response_time_mult(sg_policy); + ret = kobject_init_and_add(&tunables->attr_set.kobj, &sugov_tunables_ktype, get_governor_parent_kobj(policy), "%s", schedutil_gov.name); @@ -808,7 +915,7 @@ static int sugov_start(struct cpufreq_policy *policy) void (*uu)(struct update_util_data *data, u64 time, unsigned int flags); unsigned int cpu; - sg_policy->freq_update_delay_ns = sg_policy->tunables->rate_limit_us * NSEC_PER_USEC; + sg_policy->freq_update_delay_ns = sg_policy->tunables->rate_limit_us * NSEC_PER_USEC; sg_policy->last_freq_update_time = 0; sg_policy->next_freq = 0; sg_policy->work_in_progress = false;
The new tunable, response_time_ms, allow us to speed up or slow down the response time of the policy to meet the perf, power and thermal characteristic desired by the user/sysadmin. There's no single universal trade-off that we can apply for all systems even if they use the same SoC. The form factor of the system, the dominant use case, and in case of battery powered systems, the size of the battery and presence or absence of active cooling can play a big role on what would be best to use. The new tunable provides sensible defaults, but yet gives the power to control the response time to the user/sysadmin, if they wish to. This tunable is applied before we apply the DVFS headroom. The default behavior of applying 1.25 headroom can be re-instated easily now. But we continue to keep the min required headroom to overcome hardware limitation in its speed to change DVFS. And any additional headroom to speed things up must be applied by userspace to match their expectation for best perf/watt as it dictates a type of policy that will be better for some systems, but worse for others. There's a whitespace clean up included in sugov_start(). Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> --- Documentation/admin-guide/pm/cpufreq.rst | 17 +++- drivers/cpufreq/cpufreq.c | 4 +- include/linux/cpufreq.h | 3 + kernel/sched/cpufreq_schedutil.c | 115 ++++++++++++++++++++++- 4 files changed, 132 insertions(+), 7 deletions(-)