linux-kernel - Re: [PATCH v2 2/4] perf: Align user space counter reading with code

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Date:   Tue, 19 Jul 2022 15:42:15 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Rob Herring <robh@...nel.org>
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 v2 2/4] perf: Align user space counter reading with code

On Tue, Jul 19, 2022 at 10:45 AM Rob Herring <robh@...nel.org> wrote:
>
> On Wed, Jun 8, 2022 at 11:24 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.
> >
> > 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();
> > -        *       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 +597,24 @@ 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);
>
> I still think this chunk should stay as-is because mul_u64_u32_shr
> isn't defined here. At least a comment as to what the C version does:
>
> /* time_offset + (u64)(((unsigned __int128)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:
> > +        * enabled and possible running (if index) to improve the scaling. Due
> > +        * to event multiplexing, running maybe zero and so care is needed to
>
> s/maybe/may be/
>
> > +        * avoid division by zero.
> >          *
> >          *   enabled += delta;
> > -        *   if (index)
> > +        *   if (idx)
>
> This should remain 'index'.

Thanks, I've tried to incorporate all of this into v3:
https://lore.kernel.org/lkml/20220719223946.176299-1-irogers@google.com/

Ian

> >          *     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;