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SubjectRe: [PATCH RESEND v4] perf/core: Fix installing cgroup event into cpu
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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