Re: [External] Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 22, 2022 at 09:38:21PM +0800, Chengming Zhou wrote:
> On 2022/3/22 8:59 下午, Peter Zijlstra wrote:
> > On Tue, Mar 22, 2022 at 08:08:29PM +0800, Chengming Zhou wrote:
> >> There is a race problem that can trigger WARN_ON_ONCE(cpuctx->cgrp)
> >> in perf_cgroup_switch().
> >>
> >> CPU1					CPU2
> >> (in context_switch)			(attach running task)
> >> perf_cgroup_sched_out(prev, next)
> >> 	cgrp1 == cgrp2 is True
> >> 					next->cgroups = cgrp3
> >> 					perf_cgroup_attach()
> >> perf_cgroup_sched_in(prev, next)
> >> 	cgrp1 == cgrp3 is False
> >>
> >> The commit a8d757ef076f ("perf events: Fix slow and broken cgroup
> >> context switch code") would save cpuctx switch out/in when the
> >> perf_cgroup of "prev" and "next" are the same.
> >>
> >> But perf_cgroup of task can change in concurrent with context_switch.
> > 
> > Can you clarify? IIRC then a task changes cgroup it goes throught the
> > whole ->attach() dance, and that serializes against the context switch
> > code.
> > 
> 
> task->cgroups changed before perf_cgroup_attach(), and is not serialized
> against the context switch, since task->cgroups can be changed without
> rq lock held. (cgroup v1 or cgroup v2 with PSI disabled)
> 
> So perf_cgroup_from_task() in perf_cgroup_switch() may see the old or
> new perf_cgroup when do context switch.

__schedule()
  local_irq_disable();				<--- IRQ disable
  rq_lock();

  ...

  context_switch()
    prepare_task_switch()
      perf_event_task_sched_out()
        __perf_event_task_sched_out()
	  perf_cgroup_sched_out();

  switch_to()
  finish_task_switch()
    perf_event_task_sched_in()
      __perf_event_task_sched_in()
        perf_cgroup_sched_in();
    finish_lock_switch()
      raw_spin_irq_unlock_irq();		<--- IRQ enable


vs

perf_event_cgrp_subsys.attach = perf_cgroup_attach()
  cgroup_taskset_for_each()
    task_function_call(task, __perf_cgroup_move) <--- sends IPI


Please explain how this can interleave.