lkml.org 
[lkml]   [2022]   [Mar]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup
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);

\
 
 \ /
  Last update: 2022-09-17 16:18    [W:0.070 / U:0.944 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site