Messages in this thread | | | Date | Wed, 23 Mar 2022 14:17:44 +0100 | From | Peter Zijlstra <> | Subject | Re: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup |
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On Wed, Mar 23, 2022 at 09:07:01PM +0800, Chengming Zhou wrote: > On 2022/3/23 8:51 下午, Peter Zijlstra wrote: > > On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote: > > > >> diff --git a/kernel/events/core.c b/kernel/events/core.c > >> index 8b5cf2aedfe6..848a3bfa9513 100644 > >> --- a/kernel/events/core.c > >> +++ b/kernel/events/core.c > > > >> @@ -843,11 +845,21 @@ static void perf_cgroup_switch(struct task_struct *task) > >> */ > >> local_irq_save(flags); > >> > >> + cgrp = perf_cgroup_from_task(task, NULL); > >> + if (cgrp == __this_cpu_read(cpu_perf_cgroup)) > >> + goto out;
So this compares the cpu thing against the task thing, if matching, we bail.
> >> + > >> + __this_cpu_write(cpu_perf_cgroup, cgrp);
Then we set cpu thing.
> >> + > >> list = this_cpu_ptr(&cgrp_cpuctx_list); > >> list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) { > >> WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0); > >> > >> perf_ctx_lock(cpuctx, cpuctx->task_ctx); > >> + > >> + if (cpuctx->cgrp == cgrp) > >> + continue; > >> + > >> perf_pmu_disable(cpuctx->ctx.pmu); > >> > >> cpu_ctx_sched_out(cpuctx, EVENT_ALL);
> >> + cpuctx->cgrp = cgrp
But here we already have exactly the same pattern, we match cpuctx thing against task thing and skip/set etc.
> > Also, I really don't see the point of cpu_perf_cgroup, cpuctx->cgrp > > should be able to do this. > > But the problem is that we have two cpuctx on the percpu list, do you > mean we should use perf_cgroup of the first cpuctx to compare with > the next task's perf_cgroup ? > > Or we should delete the cgrp in cpuctx, and use this new percpu cpu_perf_cgroup?
I'm a bit confused, per the above, you already do exactly what the new cpu_perf_cgroup does on the cpuctx->cgrp variable. AFAICT the only think the new per-cpu variable does is avoid a lock, howveer:
> > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -833,6 +833,7 @@ static DEFINE_PER_CPU(struct list_head, > > */ > > static void perf_cgroup_switch(struct task_struct *task) > > { > > + struct perf_cgroup *cgrp; > > struct perf_cpu_context *cpuctx, *tmp; > > struct list_head *list; > > unsigned long flags; > > @@ -843,11 +844,20 @@ static void perf_cgroup_switch(struct ta > > */ > > local_irq_save(flags); > > > > + cgrp = perf_cgroup_from_task(task, NULL); > > + > > list = this_cpu_ptr(&cgrp_cpuctx_list); > > list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) { > > WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0); > > > > + if (READ_ONCE(cpuctx->cgrp == cgrp)) > > + continue
I think we can avoid that by doing an early check, hmm?
> > + > > perf_ctx_lock(cpuctx, cpuctx->task_ctx); > > + > > + if (cpuctx->cgrp == cgrp) > > + goto next; > > + > > perf_pmu_disable(cpuctx->ctx.pmu); > > > > cpu_ctx_sched_out(cpuctx, EVENT_ALL); > > @@ -855,50 +865,22 @@ static void perf_cgroup_switch(struct ta > > * must not be done before ctxswout due > > * to event_filter_match() in event_sched_out() > > */ > > - cpuctx->cgrp = perf_cgroup_from_task(task, > > - &cpuctx->ctx); > > + WRITE_ONCE(cpuctx->cgrp, cgrp);
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