Messages in this thread | | | Date | Fri, 9 Feb 2018 11:11:28 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RESEND v4] perf/core: Fix installing cgroup event into cpu |
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On Thu, Feb 08, 2018 at 11:33:44AM +0800, linxiulei@gmail.com wrote: > From: "leilei.lin" <leilei.lin@alibaba-inc.com> > > Do not install cgroup event into the CPU context and schedule it > if the cgroup is not running on this CPU > > While there is no task of cgroup running specified CPU, current > kernel still install cgroup event into CPU context that causes > another cgroup event can't be installed into this CPU. > > This patch prevent scheduling events at __perf_install_in_context() > and installing events at list_update_cgroup_event() if cgroup isn't > running on specified CPU. > > Signed-off-by: leilei.lin <leilei.lin@alibaba-inc.com> > --- > v2: Set cpuctx->cgrp only if the same cgroup is running on this > CPU otherwise following events couldn't be activated immediately > v3: Enhance the comments and commit message > v4: Adjust to config > > kernel/events/core.c | 50 +++++++++++++++++++++++++++++++++++++------------- > 1 file changed, 37 insertions(+), 13 deletions(-) > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 4df5b69..fd28d61 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -933,31 +933,41 @@ 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); > > + /* > + * if only the cgroup is running on this cpu, > + * we put/remove this cgroup into cpu context. > + * Or it would case mismatch in following cgroup > + * events at event_filter_match() > + */ > + if (cgroup_is_descendant(cgrp->css.cgroup, event->cgrp->css.cgroup)) { > + if (add) > cpuctx->cgrp = cgrp; > + else > + cpuctx->cgrp = NULL; > }
I am still not convinced this is correct.
Suppose we have
R / \ A B / \ C
And our current task is of B, and B has an event.
We then install an event in C, if we then destroy our event in C, it would clear cpuctx->cgrp, which is wrong, since there is still an event in B.
Simpler still, if B were to have 2 events, and we'd remove one, that would still clear cpuctx->cgrp, even though there is an event left.
This is the exact issue I pointed out last time, and I still don't see how it would now be correct.
Northing explains why its ok to have NULL cpuctx->cgrp when there are in fact still cgroup events on the CPU.
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