Re: [RFC v2] perf: Rewrite core context handling

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 13, 2022 at 04:35:11PM +0200, Peter Zijlstra wrote:

> @@ -3196,11 +3187,52 @@ static int perf_event_modify_attr(struct
>  	return err;
>  }
>  
> -static void ctx_sched_out(struct perf_event_context *ctx,
> -			  struct perf_cpu_context *cpuctx,
> -			  enum event_type_t event_type)
> +static void __pmu_ctx_sched_out(struct perf_event_pmu_context *pmu_ctx,
> +				enum event_type_t event_type)
>  {
> +	struct perf_event_context *ctx = pmu_ctx->ctx;
>  	struct perf_event *event, *tmp;
> +	struct pmu *pmu = pmu_ctx->pmu;
> +
> +	if (ctx->task && !ctx->is_active) {
> +		struct perf_cpu_pmu_context *cpc;
> +
> +		cpc = this_cpu_ptr(pmu->cpu_pmu_context);
> +		WARN_ON_ONCE(cpc->task_epc != pmu_ctx);
> +		cpc->task_epc = NULL;
> +	}
> +
> +	if (!event_type)
> +		return;
> +
> +	perf_pmu_disable(pmu);
> +	if (event_type & EVENT_PINNED) {
> +		list_for_each_entry_safe(event, tmp,
> +				&pmu_ctx->pinned_active,
> +				active_list)
> +			group_sched_out(event, ctx);
> +	}
> +
> +	if (event_type & EVENT_FLEXIBLE) {
> +		list_for_each_entry_safe(event, tmp,
> +				&pmu_ctx->flexible_active,
> +				active_list)
> +			group_sched_out(event, ctx);
> +		/*
> +		 * Since we cleared EVENT_FLEXIBLE, also clear
> +		 * rotate_necessary, is will be reset by
> +		 * ctx_flexible_sched_in() when needed.
> +		 */
> +		pmu_ctx->rotate_necessary = 0;
> +	}
> +	perf_pmu_enable(pmu);
> +}
> +
> +static void
> +ctx_sched_out(struct perf_event_context *ctx, enum event_type_t event_type)
> +{
> +	struct perf_cpu_context *cpuctx = this_cpu_ptr(&cpu_context);
> +	struct perf_event_pmu_context *pmu_ctx;
>  	int is_active = ctx->is_active;
>  
>  	lockdep_assert_held(&ctx->lock);
> @@ -3251,24 +3283,8 @@ static void ctx_sched_out(struct perf_ev
>  	if (!ctx->nr_active || !(is_active & EVENT_ALL))
>  		return;
>  
> -	perf_pmu_disable(ctx->pmu);
> -	if (is_active & EVENT_PINNED) {
> -		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
> -			group_sched_out(event, cpuctx, ctx);
> -	}
> -
> -	if (is_active & EVENT_FLEXIBLE) {
> -		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list)
> -			group_sched_out(event, cpuctx, ctx);
> -
> -		/*
> -		 * Since we cleared EVENT_FLEXIBLE, also clear
> -		 * rotate_necessary, is will be reset by
> -		 * ctx_flexible_sched_in() when needed.
> -		 */
> -		ctx->rotate_necessary = 0;
> -	}
> -	perf_pmu_enable(ctx->pmu);
> +	list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry)
> +		__pmu_ctx_sched_out(pmu_ctx, is_active);
>  }

You mentioned trouble with cpc->task_epc, there's one rebase mistake
from you and an original bug from me.

You lost the last hunk, I forgot to clear cpc on
perf_remove_from_context().

With these fixes I can run: 'perf test' without things going
insta-splat.

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2311,6 +2311,7 @@ __perf_remove_from_context(struct perf_e
 			   struct perf_event_context *ctx,
 			   void *info)
 {
+	struct perf_event_pmu_context *pmu_ctx = event->pmu_ctx;
 	unsigned long flags = (unsigned long)info;
 
 	if (ctx->is_active & EVENT_TIME) {
@@ -2325,8 +2326,17 @@ __perf_remove_from_context(struct perf_e
 		perf_child_detach(event);
 	list_del_event(event, ctx);
 
-	if (!event->pmu_ctx->nr_events)
-		event->pmu_ctx->rotate_necessary = 0;
+	if (!pmu_ctx->nr_events) {
+		pmu_ctx->rotate_necessary = 0;
+
+		if (ctx->task) {
+			struct perf_cpu_pmu_context *cpc;
+
+			cpc = this_cpu_ptr(pmu_ctx->pmu->cpu_pmu_context);
+			WARN_ON_ONCE(cpc->task_epc && cpc->task_epc != pmu_ctx);
+			cpc->task_epc = NULL;
+		}
+	}
 
 	if (!ctx->nr_events && ctx->is_active) {
 		if (ctx == &cpuctx->ctx)
@@ -3198,7 +3208,7 @@ static void __pmu_ctx_sched_out(struct p
 		struct perf_cpu_pmu_context *cpc;
 
 		cpc = this_cpu_ptr(pmu->cpu_pmu_context);
-		WARN_ON_ONCE(cpc->task_epc != pmu_ctx);
+		WARN_ON_ONCE(cpc->task_epc && cpc->task_epc != pmu_ctx);
 		cpc->task_epc = NULL;
 	}
 
@@ -3280,9 +3290,6 @@ ctx_sched_out(struct perf_event_context
 
 	is_active ^= ctx->is_active; /* changed bits */
 
-	if (!ctx->nr_active || !(is_active & EVENT_ALL))
-		return;
-
 	list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry)
 		__pmu_ctx_sched_out(pmu_ctx, is_active);
 }