The current throttling code triggers WARN below via following
workload (only hit on AMD machine with 48 CPUs):
# while [ 1 ]; do perf record perf bench sched messaging; done
WARNING: at arch/x86/kernel/cpu/perf_event.c:1054 x86_pmu_start+0xc6/0x100()
SNIP
Call Trace:
<IRQ> [<ffffffff815f62d6>] dump_stack+0x19/0x1b
[<ffffffff8105f531>] warn_slowpath_common+0x61/0x80
[<ffffffff8105f60a>] warn_slowpath_null+0x1a/0x20
[<ffffffff810213a6>] x86_pmu_start+0xc6/0x100
[<ffffffff81129dd2>] perf_adjust_freq_unthr_context.part.75+0x182/0x1a0
[<ffffffff8112a058>] perf_event_task_tick+0xc8/0xf0
[<ffffffff81093221>] scheduler_tick+0xd1/0x140
[<ffffffff81070176>] update_process_times+0x66/0x80
[<ffffffff810b9565>] tick_sched_handle.isra.15+0x25/0x60
[<ffffffff810b95e1>] tick_sched_timer+0x41/0x60
[<ffffffff81087c24>] __run_hrtimer+0x74/0x1d0
[<ffffffff810b95a0>] ? tick_sched_handle.isra.15+0x60/0x60
[<ffffffff81088407>] hrtimer_interrupt+0xf7/0x240
[<ffffffff81606829>] smp_apic_timer_interrupt+0x69/0x9c
[<ffffffff8160569d>] apic_timer_interrupt+0x6d/0x80
<EOI> [<ffffffff81129f74>] ? __perf_event_task_sched_in+0x184/0x1a0
[<ffffffff814dd937>] ? kfree_skbmem+0x37/0x90
[<ffffffff815f2c47>] ? __slab_free+0x1ac/0x30f
[<ffffffff8118143d>] ? kfree+0xfd/0x130
[<ffffffff81181622>] kmem_cache_free+0x1b2/0x1d0
[<ffffffff814dd937>] kfree_skbmem+0x37/0x90
[<ffffffff814e03c4>] consume_skb+0x34/0x80
[<ffffffff8158b057>] unix_stream_recvmsg+0x4e7/0x820
[<ffffffff814d5546>] sock_aio_read.part.7+0x116/0x130
[<ffffffff8112c10c>] ? __perf_sw_event+0x19c/0x1e0
[<ffffffff814d5581>] sock_aio_read+0x21/0x30
[<ffffffff8119a5d0>] do_sync_read+0x80/0xb0
[<ffffffff8119ac85>] vfs_read+0x145/0x170
[<ffffffff8119b699>] SyS_read+0x49/0xa0
[<ffffffff810df516>] ? __audit_syscall_exit+0x1f6/0x2a0
[<ffffffff81604a19>] system_call_fastpath+0x16/0x1b
---[ end trace 622b7e226c4a766a ]---
The reason is race in perf_event_task_tick throttling code.
The race flow (simplified code):
- perf_throttled_count is per cpu variable and is
CPU throttling flag, here starting with 0
- perf_throttled_seq is sequence/domain for allowed
count of interrupts within the tick, gets increased
each tick
on single CPU (CPU bounded event):
... workload
perf_event_task_tick:
|
| T0 inc(perf_throttled_seq)
| T1 needs_unthr = xchg(perf_throttled_count, 0) == 0
tick gets interrupted:
... event gets throttled under new seq ...
T2 last NMI comes, event is throttled - inc(perf_throttled_count)
back to tick:
| perf_adjust_freq_unthr_context:
|
| T3 unthrottling is skiped for event (needs_unthr == 0)
| T4 event is stop and started via freq adjustment
|
tick ends
... workload
... no sample is hit for event ...
perf_event_task_tick:
|
| T5 needs_unthr = xchg(perf_throttled_count, 0) != 0 (from T2)
| T6 unthrottling is done on event (interrupts == MAX_INTERRUPTS)
| event is already started (from T4) -> WARN
Fixing this by not checking needs_unthr again and thus
check all events for unthrottling.
Signed-off-by: Jiri Olsa <jo...@redhat.com>
Reported-by: Jan Stancek <jstan...@redhat.com>
Suggested-by: Peter Zijlstra <a.p.zijls...@chello.nl>
Cc: Corey Ashford <cjash...@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Namhyung Kim <namhy...@kernel.org>
Cc: Paul Mackerras <pau...@samba.org>
Cc: Peter Zijlstra <a.p.zijls...@chello.nl>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
Cc: Andi Kleen <a...@firstfloor.org>
Cc: Stephane Eranian <eran...@google.com>
---
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 928fae7..299d1f1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2709,7 +2709,7 @@ static void perf_adjust_freq_unthr_context(struct
perf_event_context *ctx,
hwc = &event->hw;
- if (needs_unthr && hwc->interrupts == MAX_INTERRUPTS) {
+ if (hwc->interrupts == MAX_INTERRUPTS) {
hwc->interrupts = 0;
perf_log_throttle(event, 1);
event->pmu->start(event, 0);
--
1.7.11.7
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