Re: [PATCH v2] perf/core: Fix installing cgroup event into cpu

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 24, 2018 at 04:32:38PM +0800, Lin Xiulei wrote:
> >>  kernel/events/core.c | 44 +++++++++++++++++++++++++++++++-------------
> >>  1 file changed, 31 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/kernel/events/core.c b/kernel/events/core.c
> >> index 4df5b69..f766b60 100644
> >> --- a/kernel/events/core.c
> >> +++ b/kernel/events/core.c
> >> @@ -933,31 +933,36 @@ list_update_cgroup_event(struct perf_event *event,
> >>  {
> >>       struct perf_cpu_context *cpuctx;
> >>       struct list_head *cpuctx_entry;
> >> +     struct perf_cgroup *cgrp;
> >>
> >>       if (!is_cgroup_event(event))
> >>               return;
> >>
> >>       /*
> >>        * Because cgroup events are always per-cpu events,
> >>        * this will always be called from the right CPU.
> >>        */
> >>       cpuctx = __get_cpu_context(ctx);
> >> +     cgrp = perf_cgroup_from_task(current, ctx);
> >>
> >> +     /* cpuctx->cgrp is NULL unless a cgroup event is running in this CPU .*/
> >> +     if (cgroup_is_descendant(cgrp->css.cgroup, event->cgrp->css.cgroup)) {
> >> +             if (add)
> >>                       cpuctx->cgrp = cgrp;
> >> +             else
> >> +                     cpuctx->cgrp = NULL;
> >>       }
> >> +
> >> +     if (add && ctx->nr_cgroups++)
> >> +             return;
> >> +     else if (!add && --ctx->nr_cgroups)
> >> +             return;
> >> +
> >> +     cpuctx_entry = &cpuctx->cgrp_cpuctx_entry;
> >> +     if (add)
> >> +             list_add(cpuctx_entry, this_cpu_ptr(&cgrp_cpuctx_list));
> >> +     else
> >> +             list_del(cpuctx_entry);
> >>  }
> >
> > I'm a little confused; you unconditionally set cpuctx->cgrp for every
> > add/delete.
> >
> > So if we have >1 cgroup events on, and we remove one, you still clear
> > cpuctx->cgrp, that seems wrong.
> >
> > Why did you change that? The Changelog doesn't include enough clues for
> > me to know what you were trying to do.
> 
> if we have > 1 cgroup events on, whenever a cgroup was really to be
> deleted, only if this cgroup is the same as the cgroup running on this
> cpu, I would clear cpuctx->cgrp.

But that might still be too early, we might still have more cgroup
events active.

What goes wrong if we leave it set?

> Here is the problem, previous version didn't set cpuctx->cgrp anymore
> if ctx->nr_cgroups > 1, which cases a second event would not be
> activated immediately because cpuctx->cgrp isn't equal to event->cgrp
> at event_filter_match()

OK, I think I can see that happening. Please clarify the Changelog and
maybe put a comment in the code as well.