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

[Peter Zijlstra <peterz@infradead.org>]

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.