linux-kernel - [PATCH v3 5/5] MIPS/Perf-events: Use unsigned delta for right shift in event update

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Message-Id: <1291022351-13152-6-git-send-email-dengcheng.zhu@gmail.com>
Date:	Mon, 29 Nov 2010 17:19:11 +0800
From:	Deng-Cheng Zhu <dengcheng.zhu@...il.com>
To:	ralf@...ux-mips.org, a.p.zijlstra@...llo.nl, fweisbec@...il.com,
	will.deacon@....com
Cc:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	wuzhangjin@...il.com, paulus@...ba.org, mingo@...e.hu,
	acme@...hat.com, dengcheng.zhu@...il.com, matt@...sole-pimps.org,
	sshtylyov@...sta.com
Subject: [PATCH v3 5/5] MIPS/Perf-events: Use unsigned delta for right shift in event update

Leverage the commit for ARM by Will Deacon:

- 446a5a8b1eb91a6990e5c8fe29f14e7a95b69132
    ARM: 6205/1: perf: ensure counter delta is treated as unsigned

    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 the delta value is treated as unsigned so that the
    right shift sets the upper bits to zero.

Acked-by: Will Deacon <will.deacon@....com>
Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@...il.com>
---
 arch/mips/kernel/perf_event.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/mips/kernel/perf_event.c b/arch/mips/kernel/perf_event.c
index 8f7d2f8..a824485 100644
--- a/arch/mips/kernel/perf_event.c
+++ b/arch/mips/kernel/perf_event.c
@@ -169,7 +169,7 @@ static void mipspmu_event_update(struct perf_event *event,
 	unsigned long flags;
 	int shift = 64 - TOTAL_BITS;
 	s64 prev_raw_count, new_raw_count;
-	s64 delta;
+	u64 delta;
 
 again:
 	prev_raw_count = local64_read(&hwc->prev_count);
-- 
1.7.1

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