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Date:   Mon, 1 Aug 2022 09:17:55 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ian Rogers <irogers@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Kajol Jain <kjain@linux.ibm.com>,
        Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        Rob Herring <robh@kernel.org>,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v3 1/3] perf: Align user space counter reading with code
Message-ID: <YufEc1i32OOa+6u8@kernel.org>
References: <20220719223946.176299-1-irogers@google.com>
 <20220719223946.176299-2-irogers@google.com>
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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@google.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