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From: Peter Zijlstra <peterz@infradead.org>
To: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
	mingo@kernel.org, alexander.shishkin@linux.intel.com,
	hpa@zytor.com, bp@alien8.de, brgerst@gmail.com,
	luto@amacapital.net, oleg@redhat.com, akpm@linux-foundation.org,
	tglx@linutronix.de, dvlasenk@redhat.com
Cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perf/urgent] perf/x86/intel: Fix PMI handling for Intel PT
Date: Wed, 24 Jun 2015 16:47:50 +0200
Message-ID: <20150624144750.GJ18673@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20150624131648.GM3644@twins.programming.kicks-ass.net>

Subject: perf,x86: Fix active_events imbalance

Commit 1b7b938f1817 ("perf/x86/intel: Fix PMI handling for Intel PT")
conditionally increments active_events in x86_add_exclusive() but
unconditionally decrements in x86_del_exclusive().

These extra decrements can lead to the situation where active_events is
zero and thus the PMI handler is 'disabled' while we have active events
on the PMU generating PMIs.

This leads to a truckload of:

  Uhhuh. NMI received for unknown reason 21 on CPU 28.
  Do you have a strange power saving mode enabled?
  Dazed and confused, but trying to continue

messages and generally messes up perf.

Remove the condition on the increment, double increment balanced by a
double decrement is perfectly fine.

Restructure the code a little bit to make the unconditional inc a bit
more natural.

Fixes: 1b7b938f1817 ("perf/x86/intel: Fix PMI handling for Intel PT")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/cpu/perf_event.c |   36 +++++++++++++-----------------------
 1 file changed, 13 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 5801a14..3658de4 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -357,34 +357,24 @@ void x86_release_hardware(void)
  */
 int x86_add_exclusive(unsigned int what)
 {
-	int ret = -EBUSY, i;
-
-	if (atomic_inc_not_zero(&x86_pmu.lbr_exclusive[what]))
-		return 0;
+	int i;
 
-	mutex_lock(&pmc_reserve_mutex);
-	for (i = 0; i < ARRAY_SIZE(x86_pmu.lbr_exclusive); i++) {
-		if (i != what && atomic_read(&x86_pmu.lbr_exclusive[i]))
-			goto out;
+	if (!atomic_inc_not_zero(&x86_pmu.lbr_exclusive[what])) {
+		mutex_lock(&pmc_reserve_mutex);
+		for (i = 0; i < ARRAY_SIZE(x86_pmu.lbr_exclusive); i++) {
+			if (i != what && atomic_read(&x86_pmu.lbr_exclusive[i]))
+				goto fail_unlock;
+		}
+		atomic_inc(&x86_pmu.lbr_exclusive[what]);
+		mutex_unlock(&pmc_reserve_mutex);
 	}
 
-	atomic_inc(&x86_pmu.lbr_exclusive[what]);
-	ret = 0;
+	atomic_inc(&active_events);
+	return 0;
 
-out:
+fail_unlock:
 	mutex_unlock(&pmc_reserve_mutex);
-
-	/*
-	 * Assuming that all exclusive events will share the PMI handler
-	 * (which checks active_events for whether there is work to do),
-	 * we can bump active_events counter right here, except for
-	 * x86_lbr_exclusive_lbr events that go through x86_pmu_event_init()
-	 * path, which already bumps active_events for them.
-	 */
-	if (!ret && what != x86_lbr_exclusive_lbr)
-		atomic_inc(&active_events);
-
-	return ret;
+	return -EBUSY;
 }
 
 void x86_del_exclusive(unsigned int what)

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-11 12:13 [PATCH 0/2] perf/x86/intel: Fixes for PT and BTS Alexander Shishkin
2015-06-11 12:13 ` [PATCH 1/2] perf/x86/intel/bts: Fix DS area sharing with x86_pmu events Alexander Shishkin
2015-06-19 17:58   ` [tip:perf/urgent] " tip-bot for Alexander Shishkin
2015-06-11 12:13 ` [PATCH 2/2] perf/x86/intel: Fix PMI handling for Intel PT Alexander Shishkin
2015-06-11 14:00   ` Peter Zijlstra
2015-06-12  9:08     ` Alexander Shishkin
2015-06-12 13:09       ` Peter Zijlstra
2015-06-19 17:58       ` [tip:perf/urgent] " tip-bot for Alexander Shishkin
2015-06-24 12:21         ` Peter Zijlstra
2015-06-24 13:16         ` Peter Zijlstra
2015-06-24 14:47           ` Peter Zijlstra [this message]
2015-07-01  6:57             ` [tip:perf/urgent] perf/x86: Fix 'active_events' imbalance tip-bot for Peter Zijlstra

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