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Subject[tip: perf/core] perf/core: Fix endless multiplex timer
The following commit has been merged into the perf/core branch of tip:

Commit-ID: 90c91dfb86d0ff545bd329d3ddd72c147e2ae198
Gitweb: https://git.kernel.org/tip/90c91dfb86d0ff545bd329d3ddd72c147e2ae198
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 05 Mar 2020 13:38:51 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 20 Mar 2020 13:06:22 +01:00

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
---
kernel/events/core.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ccf8d4f..b5a68d2 100644
--- 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;
}

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