Re: [PATCH 1/4] perf: Fix system-wide events miscounting during cgroup monitoring

[Liang, Kan]

On 4/30/2019 4:56 AM, Peter Zijlstra wrote:
> On Mon, Apr 29, 2019 at 07:44:02AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
>
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index e47ef76..039e2f2 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -795,6 +795,7 @@ struct perf_cpu_context {
> >   #ifdef CONFIG_CGROUP_PERF
> >   	struct perf_cgroup		*cgrp;
> >   	struct list_head		cgrp_cpuctx_entry;
> > +	unsigned int			cgrp_switch		:1;
>
> If you're not adding more bits, why not just keep it an int?

Yes, there is no more bits for now.
Sure, I will change it to int.


> >   #endif
> >   
> >   	struct list_head		sched_cb_entry;
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index dc7dead..388dd42 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -809,6 +809,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
> >   
> >   		perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> >   		perf_pmu_disable(cpuctx->ctx.pmu);
> > +		cpuctx->cgrp_switch = true;
> >   
> >   		if (mode & PERF_CGROUP_SWOUT) {
> >   			cpu_ctx_sched_out(cpuctx, EVENT_ALL);
> > @@ -832,6 +833,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
> >   							     &cpuctx->ctx);
> >   			cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);
> >   		}
> > +		cpuctx->cgrp_switch = false;
> >   		perf_pmu_enable(cpuctx->ctx.pmu);
> >   		perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> >   	}
>
> That is a bit of a hack...

We need an indicator to indicate the context switch, so we can apply the 
optimization.
I was thinking to use the existing variables, e.g. cpuctx->cgrp. But it 
doesn't work. We cannot tell if it's the first sched_in or in a context 
switch by checking cpuctx->cgrp.
cgrp_switch should be the simplest method.

> > @@ -2944,13 +2946,25 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> >   
> >   	perf_pmu_disable(ctx->pmu);
> >   	if (is_active & EVENT_PINNED) {
> > -		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
> > +		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list) {
> > +#ifdef CONFIG_CGROUP_PERF
> > +			/* Don't sched system-wide event when cgroup context switch */
> > +			if (cpuctx->cgrp_switch && !event->cgrp)
> > +				continue;
> > +#endif
> >   			group_sched_out(event, cpuctx, ctx);
> > +		}
> >   	}
>
> This works by accident, however..
>
> >   
> >   	if (is_active & EVENT_FLEXIBLE) {
> > -		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list)
> > +		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list) {
> > +#ifdef CONFIG_CGROUP_PERF
> > +			/* Don't sched system-wide event when cgroup context switch */
> > +			if (cpuctx->cgrp_switch && !event->cgrp)
> > +				continue;
> > +#endif
> >   			group_sched_out(event, cpuctx, ctx);
> > +		}
> >   	}
> >   	perf_pmu_enable(ctx->pmu);
> >   }
>
> this one is just wrong afaict.
>
> Suppose the new cgroup has pinned events, which we cannot schedule
> because you left !cgroup flexible events on.

Right, the priority order may be changed.

I think we can track the event type in perf_cgroup.
If the new cgroup has pinned events, we can fall back to slow path, 
which will switch everything.

How about the patch as below? (Not test yet)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 7eff286..f751852 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -843,6 +843,8 @@ struct perf_cgroup {
 	struct perf_cgroup_info	__percpu *info;
 	/* perf cgroup ID = (CPU ID << 32) | css subsys-unique ID */
 	u64 __percpu			*cgrp_id;
+
+	int				cgrp_event_type;
 };

 /*
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2066322..557d371 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -839,6 +839,22 @@ static void perf_cgroup_switch(struct task_struct 
*task, int mode)
 			 */
 			cpuctx->cgrp = perf_cgroup_from_task(task,
 							     &cpuctx->ctx);
+
+			/*
+			 * The system-wide events weren't sched out.
+			 * To keep the following priority order:
+			 * cpu pinned, cpu flexible
+			 * We need to switch system-wide events as well,
+			 * if the new cgroup has pinned events.
+			 *
+			 * Disable fast path and sched out the flexible events.
+			 */
+			if (cpuctx->cgrp->cgrp_event_type & EVENT_PINNED) {
+				/* disable fast path */
+				cpuctx->cgrp_switch = false;
+				cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
+			}
+
 			cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);
 		}
 		cpuctx->cgrp_switch = false;
@@ -924,6 +940,11 @@ static inline int perf_cgroup_connect(int fd, 
struct perf_event *event,
 	cgrp = container_of(css, struct perf_cgroup, css);
 	event->cgrp = cgrp;

+	if (event->attr.pinned)
+		cgrp->cgrp_event_type |= EVENT_PINNED;
+	else
+		cgrp->cgrp_event_type |= EVENT_FLEXIBLE;
+
 	cgrp_id = ((u64)smp_processor_id() << 32) | css->id;
 	event->cgrp_id = cgrp_id;
 	*per_cpu_ptr(cgrp->cgrp_id, event->cpu) = cgrp_id;


Thanks,
Kan