[patch] perf_event_open.2: update time_shift sample code

[patch] perf_event_open.2: update time_shift sample code

[Vince Weaver]

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 <vincent.weaver-e7X0jjDqjFGHXe+LvDLADg@public.gmane.org>

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

[Michael Kerrisk (man-pages)]

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 <vincent.weaver@maine.edu>
> 
> 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/