perf/core: Fix endless multiplex timer

commit 90c91dfb86d0ff545bd329d3ddd72c147e2ae198 upstream.

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>
Cc: Wen Yang <wenyang@linux.alibaba.com>
Link: https://lkml.kernel.org/r/20200305123851.GX2596@hirez.programming.kicks-ass.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d3be2cd..2f3e892 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2086,6 +2086,7 @@ __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;
@@ -2952,12 +2953,6 @@ 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)
@@ -2967,6 +2962,13 @@ 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);
 }
@@ -3705,6 +3707,12 @@ 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;
 }