perf/core: Fix endless multiplex timer

Kan and Andi reported that we fail to kill rotation when the flexible
events go empty, but the context does not. XXX moar

Fixes: fd7d55172d1e ("perf/cgroups: Don't rotate events for cgroups unnecessarily")
Reported-by: Andi Kleen <ak@linux.intel.com>
Reported-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200305123851.GX2596@hirez.programming.kicks-ass.net

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ccf8d4fc6374e4563852f6e857d38a2f63eacd56..b5a68d26f0b911c098825b2c18257c7ed85dcb02 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2291,6 +2291,7 @@ __perf_remove_from_context(struct perf_event *event,
 
        if (!ctx->nr_events && ctx->is_active) {
                ctx->is_active = 0;
+               ctx->rotate_necessary = 0;
                if (ctx->task) {
                        WARN_ON_ONCE(cpuctx->task_ctx != ctx);
                        cpuctx->task_ctx = NULL;
@@ -3188,12 +3189,6 @@ static void ctx_sched_out(struct perf_event_context *ctx,
        if (!ctx->nr_active || !(is_active & EVENT_ALL))
                return;
 
-       /*
-        * If we had been multiplexing, no rotations are necessary, now no events
-        * are active.
-        */
-       ctx->rotate_necessary = 0;
-
        perf_pmu_disable(ctx->pmu);
        if (is_active & EVENT_PINNED) {
                list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
@@ -3203,6 +3198,13 @@ static void ctx_sched_out(struct perf_event_context *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);
 }
@@ -3985,6 +3987,12 @@ ctx_event_to_rotate(struct perf_event_context *ctx)
                                      typeof(*event), group_node);
        }
 
+       /*
+        * Unconditionally clear rotate_necessary; if ctx_flexible_sched_in()
+        * finds there are unschedulable events, it will set it again.
+        */
+       ctx->rotate_necessary = 0;
+
        return event;
 }