linux-kernel - [patch] perf_event_open.2: update time

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Message-ID: <alpine.DEB.2.20.1610210036000.3240@macbook-air>
Date:   Fri, 21 Oct 2016 00:38:06 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
cc:     linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
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 <vincent.weaver@...ne.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