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Date: Fri, 16 Oct 2015 16:24:05 +0300 From: Adrian Hunter <adrian.hunter@...el.com> To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Andy Lutomirski <luto@...capital.net>, Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org, Stephane Eranian <eranian@...gle.com>, Andi Kleen <ak@...ux.intel.com> Subject: [PATCH 1/1] perf/x86: Fix time_shift in perf_event_mmap_page Commit b20112edeadf ("perf/x86: Improve accuracy of perf/sched clock") allowed the time_shift value in perf_event_mmap_page to be as much as 32. Unfortunately the documented algorithms for using time_shift have it shifting an integer, whereas to work correctly with the value 32, the type must be u64. Fix by limiting the shift to 31 and adjusting the multiplier accordingly. Also update the documentation of perf_event_mmap_page so that new code based on it will be more future-proof. Fixes: b20112edeadf ("perf/x86: Improve accuracy of perf/sched clock") Signed-off-by: Adrian Hunter <adrian.hunter@...el.com> --- arch/x86/kernel/tsc.c | 11 +++++++++++ include/uapi/linux/perf_event.h | 4 ++-- 2 files changed, 13 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 69b84a26ea17..c7c4d9c51e99 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -259,6 +259,17 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu) clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, cpu_khz, NSEC_PER_MSEC, 0); + /* + * cyc2ns_shift is exported via arch_perf_update_userpage() where it is + * not expected to be greater than 31 due to the original published + * conversion algorithm shifting a 32-bit value (now specifies a 64-bit + * value) - refer perf_event_mmap_page documentation in perf_event.h. + */ + if (data->cyc2ns_shift == 32) { + data->cyc2ns_shift = 31; + data->cyc2ns_mul >>= 1; + } + data->cyc2ns_offset = ns_now - mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, data->cyc2ns_shift); diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h index 2881145cda86..6c72e72e975c 100644 --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -476,7 +476,7 @@ struct perf_event_mmap_page { * 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); * @@ -507,7 +507,7 @@ struct perf_event_mmap_page { * And vice versa: * * 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); */ -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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