Date: Thu, 10 Mar 2016 15:39:24 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: mingo@kernel.org, alexander.shishkin@linux.intel.com, eranian@google.com
Cc: linux-kernel@vger.kernel.org, vince@deater.net, dvyukov@google.com,
        andi@firstfloor.org, jolsa@redhat.com, panand@redhat.com,
        sasha.levin@oracle.com, oleg@redhat.com,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 00/12] perf: more fixes
Message-ID: <20160310143924.GR6356@twins.programming.kicks-ass.net>
References: <20160224174539.570749654@infradead.org>
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On Wed, Feb 24, 2016 at 06:45:39PM +0100, Peter Zijlstra wrote:

> With these patches syz-kaller can still trigger some fail; most notably some
> NMI watchdog triggers and a very sporadic unthrottle bug (much like last time).

So the below seems to make the sporadic unthrottle thing much less
likely in that I haven't seen it in several hours, my machine keeps
dying on NMI watchdog bits.

Boris, who has been running syz-kaller on AMD hardware and was hitting a
very similar bug with the AMD-IBS code, says its not fixed it for him,
so maybe there's still more to find.

---
Subject: perf: Fix unthrottle

Its possible to IOC_PERIOD while the event is throttled, this would
re-start the event and the next tick would then try to unthrottle it,
and find the event wasn't actually stopped anymore.

This would tickle a WARN in the x86-pmu code which isn't expecting to
start a !stopped event.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 712570dddacd..d39477390415 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4210,6 +4210,14 @@ static void __perf_event_period(struct perf_event *event,
 	active = (event->state == PERF_EVENT_STATE_ACTIVE);
 	if (active) {
 		perf_pmu_disable(ctx->pmu);
+		/*
+		 * We could be throttled; unthrottle now to avoid the tick
+		 * trying to unthrottle while we already re-started the event.
+		 */
+		if (event->hw.interrupts == MAX_INTERRUPTS) {
+			event->hw.interrupts = 0;
+			perf_log_throttle(event, 1);
+		}
 		event->pmu->stop(event, PERF_EF_UPDATE);
 	}