2016-10-21 04:38:11

by Vince Weaver

Subject: [patch] perf_event_open.2: update time_shift sample code

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).

Signed-off-by: Vince Weaver <[email protected]>

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

2016-10-21 06:45:30

by Michael Kerrisk (man-pages)

Subject: Re: [patch] perf_event_open.2: update time_shift sample code

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


> Signed-off-by: Vince Weaver <[email protected]>
>
> 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/