Re: [patch] perf_event_open.2: update time_shift sample code
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From: Michael Kerrisk (man-pages) <hidden>
Date: 2016-10-21 06:45:30
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On 10/21/2016 06:38 AM, Vince Weaver wrote:
Linux 4.3 (b20112edeadf0b8a1416de061caa4beb11539902) improved the accuracy of the clock/ns conversion routines. As a result the shift factor can now be 32. This value is directly exported in the perf_event_open() mmap page, and this potentially breaks the sample code that shifts 1 left by the shift value. Add a cast in the sample code so that a proper 64-bit value results from the shift. This is the same change that was made to the sample code in include/uapi/linux/perf_event.h in Linux 4.4 (b9511cd761faafca7a1acc059e792c1399f9d7c6).
Thanks, Vince. Applied. Cheers, Michael
quoted hunk ↗ jump to hunk
Signed-off-by: Vince Weaver <redacted>diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2 index ae15cc6..fade28c 100644 --- a/man2/perf_event_open.2 +++ b/man2/perf_event_open.2@@ -1619,7 +1619,7 @@ delta since u64 quot, rem; u64 delta; quot = (cyc >> time_shift); - rem = cyc & ((1 << time_shift) \- 1); + rem = cyc & (((u64)1 << time_shift) \- 1); delta = time_offset + quot * time_mult + ((rem * time_mult) >> time_shift); .fi@@ -1664,7 +1664,7 @@ And vice versa: .nf quot = cyc >> time_shift; - rem = cyc & ((1 << time_shift) - 1); + rem = cyc & (((u64)1 << time_shift) - 1); timestamp = time_zero + quot * time_mult + ((rem * time_mult) >> time_shift); .fi
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/