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Date: Mon, 1 Aug 2022 09:17:55 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
Ian Rogers <irogers@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Kajol Jain <kjain@...ux.ibm.com>,
Andi Kleen <ak@...ux.intel.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Anshuman Khandual <anshuman.khandual@....com>,
linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
Rob Herring <robh@...nel.org>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v3 1/3] perf: Align user space counter reading with code
Em Tue, Jul 19, 2022 at 03:39:44PM -0700, Ian Rogers escreveu:
> Align the user space counter reading documentation with the code in
> perf_mmap__read_self. Previously the documentation was based on the perf
> rdpmc test, but now general purpose code is provided by libperf.
Peter, can you merge this so as not to make Linus raise eyebrows with me
processing things outside tools/perf/ when asking him to pull perf
userspace?
- Arnaldo
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
> include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
> tools/include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
> 2 files changed, 44 insertions(+), 26 deletions(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index d37629dbad72..6826dabb7e03 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -538,9 +538,13 @@ struct perf_event_mmap_page {
> *
> * if (pc->cap_usr_time && enabled != running) {
> * cyc = rdtsc();
> - * time_offset = pc->time_offset;
> * time_mult = pc->time_mult;
> * time_shift = pc->time_shift;
> + * time_offset = pc->time_offset;
> + * if (pc->cap_user_time_short) {
> + * time_cycles = pc->time_cycles;
> + * time_mask = pc->time_mask;
> + * }
> * }
> *
> * index = pc->index;
> @@ -548,6 +552,9 @@ struct perf_event_mmap_page {
> * if (pc->cap_user_rdpmc && index) {
> * width = pc->pmc_width;
> * pmc = rdpmc(index - 1);
> + * pmc <<= 64 - width;
> + * pmc >>= 64 - width;
> + * count += pmc;
> * }
> *
> * barrier();
> @@ -590,25 +597,27 @@ struct perf_event_mmap_page {
> * If cap_usr_time the below fields can be used to compute the time
> * delta since time_enabled (in ns) using rdtsc or similar.
> *
> - * u64 quot, rem;
> - * u64 delta;
> - *
> - * quot = (cyc >> time_shift);
> - * rem = cyc & (((u64)1 << time_shift) - 1);
> - * delta = time_offset + quot * time_mult +
> - * ((rem * time_mult) >> time_shift);
> + * cyc = time_cycles + ((cyc - time_cycles) & time_mask);
> + * delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
> *
> * Where time_offset,time_mult,time_shift and cyc are read in the
> - * seqcount loop described above. This delta can then be added to
> - * enabled and possible running (if index), improving the scaling:
> + * seqcount loop described above. mul_u64_u32_shr will compute:
> + *
> + * (u64)(((unsigned __int128)cyc * time_mult) >> time_shift)
> + *
> + * This delta can then be added to enabled and possible running (if
> + * index) to improve the scaling. Due to event multiplexing, running
> + * may be zero and so care is needed to avoid division by zero.
> *
> * enabled += delta;
> * if (index)
> * running += delta;
> *
> - * quot = count / running;
> - * rem = count % running;
> - * count = quot * enabled + (rem * enabled) / running;
> + * if (running != 0) {
> + * quot = count / running;
> + * rem = count % running;
> + * count = quot * enabled + (rem * enabled) / running;
> + * }
> */
> __u16 time_shift;
> __u32 time_mult;
> diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
> index d37629dbad72..6826dabb7e03 100644
> --- a/tools/include/uapi/linux/perf_event.h
> +++ b/tools/include/uapi/linux/perf_event.h
> @@ -538,9 +538,13 @@ struct perf_event_mmap_page {
> *
> * if (pc->cap_usr_time && enabled != running) {
> * cyc = rdtsc();
> - * time_offset = pc->time_offset;
> * time_mult = pc->time_mult;
> * time_shift = pc->time_shift;
> + * time_offset = pc->time_offset;
> + * if (pc->cap_user_time_short) {
> + * time_cycles = pc->time_cycles;
> + * time_mask = pc->time_mask;
> + * }
> * }
> *
> * index = pc->index;
> @@ -548,6 +552,9 @@ struct perf_event_mmap_page {
> * if (pc->cap_user_rdpmc && index) {
> * width = pc->pmc_width;
> * pmc = rdpmc(index - 1);
> + * pmc <<= 64 - width;
> + * pmc >>= 64 - width;
> + * count += pmc;
> * }
> *
> * barrier();
> @@ -590,25 +597,27 @@ struct perf_event_mmap_page {
> * If cap_usr_time the below fields can be used to compute the time
> * delta since time_enabled (in ns) using rdtsc or similar.
> *
> - * u64 quot, rem;
> - * u64 delta;
> - *
> - * quot = (cyc >> time_shift);
> - * rem = cyc & (((u64)1 << time_shift) - 1);
> - * delta = time_offset + quot * time_mult +
> - * ((rem * time_mult) >> time_shift);
> + * cyc = time_cycles + ((cyc - time_cycles) & time_mask);
> + * delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
> *
> * Where time_offset,time_mult,time_shift and cyc are read in the
> - * seqcount loop described above. This delta can then be added to
> - * enabled and possible running (if index), improving the scaling:
> + * seqcount loop described above. mul_u64_u32_shr will compute:
> + *
> + * (u64)(((unsigned __int128)cyc * time_mult) >> time_shift)
> + *
> + * This delta can then be added to enabled and possible running (if
> + * index) to improve the scaling. Due to event multiplexing, running
> + * may be zero and so care is needed to avoid division by zero.
> *
> * enabled += delta;
> * if (index)
> * running += delta;
> *
> - * quot = count / running;
> - * rem = count % running;
> - * count = quot * enabled + (rem * enabled) / running;
> + * if (running != 0) {
> + * quot = count / running;
> + * rem = count % running;
> + * count = quot * enabled + (rem * enabled) / running;
> + * }
> */
> __u16 time_shift;
> __u32 time_mult;
> --
> 2.37.0.170.g444d1eabd0-goog
--
- Arnaldo
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