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

[Peter Zijlstra]

On Wed, Jan 24, 2018 at 03:50:10PM +0800, linxiulei@xxxxxxxxx wrote:
> From: "leilei.lin" <leilei.lin@xxxxxxxxxxxxxxx>
> 
> Do not install cgroup event into the CPU context 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 changelog doesn't really cover the extend of the changes done.


> Signed-off-by: leilei.lin <leilei.lin@xxxxxxxxxxxxxxx>
> ---
>  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.