| Message ID | 1391728237-4441-3-git-send-email-daniel.lezcano@linaro.org |
|---|---|
| State | Accepted |
| Commit | e5fc66119ec97054eefc83f173a7ee9e133c3c3a |
| Headers | show |
On 02/10/2014 10:24 AM, Preeti Murthy wrote: > HI Daniel, > > Isn't the only scenario where another cpu can put an idle task on > our runqueue, Well, I am not sure to understand what you meant, but I assume you are asking if it is possible to have a task to be pulled when we are idle, right ? This patch fixes the race when the current cpu is *about* to enter idle when calling schedule(). > in nohz_idle_balance() where only the cpus in > the nohz.idle_cpus_mask are iterated through. But for the case > that this patch is addressing, the cpu in question is not yet a part > of the nohz.idle_cpus_mask right? > > Any other case would trigger load balancing on the same cpu, but > we are preempt_disabled and interrupt disabled at this point. > > Thanks > > Regards > Preeti U Murthy > > On Fri, Feb 7, 2014 at 4:40 AM, Daniel Lezcano > <daniel.lezcano@linaro.org> wrote: >> The scheduler main function 'schedule()' checks if there are no more tasks >> on the runqueue. Then it checks if a task should be pulled in the current >> runqueue in idle_balance() assuming it will go to idle otherwise. >> >> But the idle_balance() releases the rq->lock in order to lookup in the sched >> domains and takes the lock again right after. That opens a window where >> another cpu may put a task in our runqueue, so we won't go to idle but >> we have filled the idle_stamp, thinking we will. >> >> This patch closes the window by checking if the runqueue has been modified >> but without pulling a task after taking the lock again, so we won't go to idle >> right after in the __schedule() function. >> >> Cc: alex.shi@linaro.org >> Cc: peterz@infradead.org >> Cc: mingo@kernel.org >> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> >> Signed-off-by: Peter Zijlstra <peterz@infradead.org> >> --- >> kernel/sched/fair.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 428bc9d..5ebc681 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6589,6 +6589,13 @@ void idle_balance(struct rq *this_rq) >> >> raw_spin_lock(&this_rq->lock); >> >> + /* >> + * While browsing the domains, we released the rq lock. >> + * A task could have be enqueued in the meantime >> + */ >> + if (this_rq->nr_running && !pulled_task) >> + return; >> + >> if (pulled_task || time_after(jiffies, this_rq->next_balance)) { >> /* >> * We are going idle. next_balance may be set based on >> -- >> 1.7.9.5 >> >> -- >> 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 02/11/2014 07:11 PM, Daniel Lezcano wrote: > On 02/10/2014 10:24 AM, Preeti Murthy wrote: >> HI Daniel, >> >> Isn't the only scenario where another cpu can put an idle task on >> our runqueue, > > Well, I am not sure to understand what you meant, but I assume you are > asking if it is possible to have a task to be pulled when we are idle, > right ? > > This patch fixes the race when the current cpu is *about* to enter idle > when calling schedule(). Preeti said the she didn't see a possible to insert a task on the cpu. I also did a quick check, maybe task come from wakeup path?
On 02/07/2014 07:10 AM, Daniel Lezcano wrote: > The scheduler main function 'schedule()' checks if there are no more tasks > on the runqueue. Then it checks if a task should be pulled in the current > runqueue in idle_balance() assuming it will go to idle otherwise. > > But the idle_balance() releases the rq->lock in order to lookup in the sched > domains and takes the lock again right after. That opens a window where > another cpu may put a task in our runqueue, so we won't go to idle but > we have filled the idle_stamp, thinking we will. > > This patch closes the window by checking if the runqueue has been modified > but without pulling a task after taking the lock again, so we won't go to idle > right after in the __schedule() function. > > Cc: alex.shi@linaro.org > Cc: peterz@infradead.org > Cc: mingo@kernel.org > Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> > Signed-off-by: Peter Zijlstra <peterz@infradead.org> > --- > kernel/sched/fair.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 428bc9d..5ebc681 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6589,6 +6589,13 @@ void idle_balance(struct rq *this_rq) > > raw_spin_lock(&this_rq->lock); > > + /* > + * While browsing the domains, we released the rq lock. > + * A task could have be enqueued in the meantime > + */ Mind to move the following line up to here? if (curr_cost > this_rq->max_idle_balance_cost) this_rq->max_idle_balance_cost = curr_cost; > + if (this_rq->nr_running && !pulled_task) > + return; > + > if (pulled_task || time_after(jiffies, this_rq->next_balance)) { > /* > * We are going idle. next_balance may be set based on >
On 02/13/2014 11:10 AM, Preeti U Murthy wrote: > Hi, > > On 02/13/2014 01:15 PM, Alex Shi wrote: >> On 02/11/2014 07:11 PM, Daniel Lezcano wrote: >>> On 02/10/2014 10:24 AM, Preeti Murthy wrote: >>>> HI Daniel, >>>> >>>> Isn't the only scenario where another cpu can put an idle task on >>>> our runqueue, >>> >>> Well, I am not sure to understand what you meant, but I assume you are >>> asking if it is possible to have a task to be pulled when we are idle, >>> right ? >>> >>> This patch fixes the race when the current cpu is *about* to enter idle >>> when calling schedule(). >> >> Preeti said the she didn't see a possible to insert a task on the cpu. >> >> I also did a quick check, maybe task come from wakeup path? > > Yes this is possible. Thanks for pointing this :) > > Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com> Thanks for the review ! -- Daniel
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 428bc9d..5ebc681 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6589,6 +6589,13 @@ void idle_balance(struct rq *this_rq) raw_spin_lock(&this_rq->lock); + /* + * While browsing the domains, we released the rq lock. + * A task could have be enqueued in the meantime + */ + if (this_rq->nr_running && !pulled_task) + return; + if (pulled_task || time_after(jiffies, this_rq->next_balance)) { /* * We are going idle. next_balance may be set based on