linux-kernel - Re: [PATCH 2/4] perf: Align user space counter reading with code

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Message-ID: <CAL_Jsq+rWB7AYr4THs2Dg98ss8cuL38QVuvTbQNwar2ePqwuww@mail.gmail.com>
Date:   Wed, 8 Jun 2022 17:24:48 -0600
From:   Rob Herring <robh@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        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-kernel@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/4] perf: Align user space counter reading with code

On Wed, Jun 8, 2022 at 4:44 PM Ian Rogers <irogers@...gle.com> wrote:
>
> 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.

IMO, this copy of not quite right code should just be removed perhaps
with a pointer to perf_mmap__read_self(). It will just get out of date
again. For example, the read loop might get rewritten with restartable
sequences.

> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  include/uapi/linux/perf_event.h       | 32 ++++++++++++++++-----------
>  tools/include/uapi/linux/perf_event.h | 32 ++++++++++++++++-----------
>  2 files changed, 38 insertions(+), 26 deletions(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index d37629dbad72..3b84e0ad0723 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();

Kind of x86 specific.

> -        *       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;
> +        *       }

This still misses the need for READ_ONCE().

>          *     }
>          *
>          *     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 +597,24 @@ 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);

For documentation purposes, the original code was easier to read and
this is just an optimization. What does mul_u64_u32_shr() do exactly?
It's not documented.

>          *
>          * 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:
> +        * enabled and possible running (if index) to improve the scaling. Due
> +        * to event multiplexing, running maybe zero and so care is needed to
> +        * avoid division by zero.
>          *
>          *   enabled += delta;
> -        *   if (index)
> +        *   if (idx)
>          *     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;