This patch fixes a bug introduced by:
commit 3cbaa59069677920186dcf502632ca1df4329f80
Author: Peter Zijlstra <[email protected]>
Date: Wed Feb 24 18:45:47 2016 +0100
perf: Fix ctx time tracking by introducing EVENT_TIME
This patch introduce a bug in the time tracking of events when multiplexing is
used.
The issue is easily reproducible with the following perf run:
$ perf stat -a -C 0 -e branches,branches,branches,branches,branches -I 1000
1.000730239 652,394 branches (66.41%)
1.000730239 597,809 branches (66.41%)
1.000730239 593,870 branches (66.63%)
1.000730239 651,440 branches (67.03%)
1.000730239 656,725 branches (66.96%)
1.000730239 <not counted> branches
One branches event is shown as not having run. Yet, with multiplexing, all
events
should run especially with a 1s (-I 1000) interval. The delta for time_running
comes out to 0. Yet, the event has run because the kernel is actually
multiplexing
the events. The problem is that the time tracking is the kernel and especially
in
ctx_sched_out() is wrong now.
The problem is that in case that the kernel enters ctx_sched_out() with the
following state:
ctx->is_active=0x7 event_type=0x1
Call Trace:
[<ffffffff813ddd41>] dump_stack+0x63/0x82
[<ffffffff81182bdc>] ctx_sched_out+0x2bc/0x2d0
[<ffffffff81183896>] perf_mux_hrtimer_handler+0xf6/0x2c0
[<ffffffff811837a0>] ? __perf_install_in_context+0x130/0x130
[<ffffffff810f5818>] __hrtimer_run_queues+0xf8/0x2f0
[<ffffffff810f6097>] hrtimer_interrupt+0xb7/0x1d0
[<ffffffff810509a8>] local_apic_timer_interrupt+0x38/0x60
[<ffffffff8175ca9d>] smp_apic_timer_interrupt+0x3d/0x50
[<ffffffff8175ac7c>] apic_timer_interrupt+0x8c/0xa0
In that case, the test:
if (is_active & EVENT_TIME)
will be false and the time will not be updated. Time must always be updated on
sched out. This patch fixes the problem.
Patch is relative to PeterZ queue.tip perf/core branch.
Signed-off-by: Stephane Eranian <[email protected]>
---
kernel/events/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 339aa46..4ba41f6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2447,7 +2447,7 @@ static void ctx_sched_out(struct perf_event_context *ctx,
is_active ^= ctx->is_active; /* changed bits */
- if (is_active & EVENT_TIME) {
+ if (ctx->is_active & EVENT_TIME) {
/* update (and stop) ctx time */
update_context_time(ctx);
update_cgrp_time_from_cpuctx(cpuctx);
--
2.5.0
