From: will.deacon@arm•com (Will Deacon)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH] ARM: perf: ensure counter delta is limited to 32-bits
Date: Fri, 2 Jul 2010 13:44:02 +0100 [thread overview]
Message-ID: <1278074642-3045-1-git-send-email-will.deacon@arm.com> (raw)
Hardware performance counters on ARM are 32-bits wide but
atomic64_t variables are used to represent counter data
in the hw_perf_event structure.
The armpmu_event_update function right-shifts a signed
64-bit delta variable and adds the result to the event count.
This can lead to shifting in sign-bits if the MSB of the
32-bit counter value is set. This results in perf output such as:
Performance counter stats for 'sleep 20':
18446744073460670464 cycles <-- 0xFFFFFFFFF12A6000
7783773 instructions # 0.000 IPC
465 context-switches
161 page-faults
1172393 branches
20.154242147 seconds time elapsed
This patch ensures that only the bottom 32 bits of the delta
value are used.
Cc: Jamie Iles <jamie.iles@picochip•com>
Signed-off-by: Will Deacon <will.deacon@arm•com>
---
arch/arm/kernel/perf_event.c | 7 ++-----
1 files changed, 2 insertions(+), 5 deletions(-)
diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index c457686..c7f7a14 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -199,9 +199,7 @@ armpmu_event_update(struct perf_event *event,
struct hw_perf_event *hwc,
int idx)
{
- int shift = 64 - 32;
- s64 prev_raw_count, new_raw_count;
- s64 delta;
+ s64 prev_raw_count, new_raw_count, delta;
again:
prev_raw_count = atomic64_read(&hwc->prev_count);
@@ -211,8 +209,7 @@ again:
new_raw_count) != prev_raw_count)
goto again;
- delta = (new_raw_count << shift) - (prev_raw_count << shift);
- delta >>= shift;
+ delta = (new_raw_count - prev_raw_count) & 0xffffffff;
atomic64_add(delta, &event->count);
atomic64_sub(delta, &hwc->period_left);
--
1.6.3.3
next reply other threads:[~2010-07-02 12:44 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-02 12:44 Will Deacon [this message]
2010-07-02 18:05 ` [PATCH] ARM: perf: ensure counter delta is limited to 32-bits Jamie Iles
2010-07-02 13:22 ` Will Deacon
2010-07-02 13:38 ` Will Deacon
2010-07-02 18:48 ` Jamie Iles
2010-07-02 14:17 ` Will Deacon
2010-07-02 20:04 ` Jamie Iles
2010-07-02 15:36 ` Will Deacon
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