Messages in this thread | | | Date | Thu, 14 Jul 2022 13:37:02 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/3] sched/core: Fix the bug that task won't enqueue into core tree when update cookie |
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On Mon, Jul 04, 2022 at 10:53:20AM +0200, Peter Zijlstra wrote: > On Tue, Jun 28, 2022 at 03:57:23PM +0800, Cruz Zhao wrote: > > In function sched_core_update_cookie(), a task will enqueue into the > > core tree only when it enqueued before, that is, if an uncookied task > > is cookied, it will not enqueue into the core tree until it enqueue > > again, which will result in unnecessary force idle. > > > > Here follows the scenario: > > CPU x and CPU y are a pair of SMT siblings. > > 1. Start task a running on CPU x without sleeping, and task b and > > task c running on CPU y without sleeping. > > 2. We create a cookie and share it to task a and task b, and then > > we create another cookie and share it to task c. > > 3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched > > > > And we will find out that core_forceidle_sum of task a takes 30% > > time of the sampling period, which shouldn't happen as task a and b > > have the same cookie. > > > > Then we migrate task a to CPU x', migrate task b and c to CPU y', where > > CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we > > will found out that core_forceidle_sum of task a and b are almost zero. > > > > To solve this problem, we enqueue the task into the core tree if it's > > on rq. > > > > Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management") > > Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com> > > --- > > kernel/sched/core_sched.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c > > index 38a2cec..ba2466c 100644 > > --- a/kernel/sched/core_sched.c > > +++ b/kernel/sched/core_sched.c > > @@ -75,7 +75,7 @@ static unsigned long sched_core_update_cookie(struct task_struct *p, > > old_cookie = p->core_cookie; > > p->core_cookie = cookie; > > > > - if (enqueued) > > + if (task_on_rq_queued(p)) > > sched_core_enqueue(rq, p); > > > > /* > > Yeah; I suppose that's true. However if we want to consider the > asymmetric case, we should be complete and also consider the case where > we clear the cookie. > > And if you remove the second use of @enqueued, having that variable is > rather redudant, which then leaves me with something like this.
Can you please confirm the below works for you so I can queue it?
> --- > Subject: sched/core: Fix the bug that task won't enqueue into core tree when update cookie > From: Cruz Zhao <CruzZhao@linux.alibaba.com> > Date: Tue, 28 Jun 2022 15:57:23 +0800 > > From: Cruz Zhao <CruzZhao@linux.alibaba.com> > > In function sched_core_update_cookie(), a task will enqueue into the > core tree only when it enqueued before, that is, if an uncookied task > is cookied, it will not enqueue into the core tree until it enqueue > again, which will result in unnecessary force idle. > > Here follows the scenario: > CPU x and CPU y are a pair of SMT siblings. > 1. Start task a running on CPU x without sleeping, and task b and > task c running on CPU y without sleeping. > 2. We create a cookie and share it to task a and task b, and then > we create another cookie and share it to task c. > 3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched > > And we will find out that core_forceidle_sum of task a takes 30% > time of the sampling period, which shouldn't happen as task a and b > have the same cookie. > > Then we migrate task a to CPU x', migrate task b and c to CPU y', where > CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we > will found out that core_forceidle_sum of task a and b are almost zero. > > To solve this problem, we enqueue the task into the core tree if it's > on rq. > > Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management") > Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Link: https://lkml.kernel.org/r/1656403045-100840-2-git-send-email-CruzZhao@linux.alibaba.com > --- > kernel/sched/core_sched.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > --- a/kernel/sched/core_sched.c > +++ b/kernel/sched/core_sched.c > @@ -56,7 +56,6 @@ static unsigned long sched_core_update_c > unsigned long old_cookie; > struct rq_flags rf; > struct rq *rq; > - bool enqueued; > > rq = task_rq_lock(p, &rf); > > @@ -68,14 +67,16 @@ static unsigned long sched_core_update_c > */ > SCHED_WARN_ON((p->core_cookie || cookie) && !sched_core_enabled(rq)); > > - enqueued = sched_core_enqueued(p); > - if (enqueued) > + if (sched_core_enqueued(p)) > sched_core_dequeue(rq, p, DEQUEUE_SAVE); > > old_cookie = p->core_cookie; > p->core_cookie = cookie; > > - if (enqueued) > + /* > + * Consider the cases: !prev_cookie and !cookie. > + */ > + if (cookie && task_on_rq_queued(p)) > sched_core_enqueue(rq, p); > > /*
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