linux-kernel - Re: [PATCH 5/8] perf/hw_breakpoint: Remove useless code related to flexible breakpoints

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Date:   Thu, 9 Jun 2022 15:41:26 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, x86@...nel.org,
        linux-sh@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] perf/hw_breakpoint: Remove useless code related to
 flexible breakpoints

On Thu, 9 Jun 2022 at 14:04, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@...gle.com> wrote:
> >
> > Flexible breakpoints have never been implemented, with
> > bp_cpuinfo::flexible always being 0. Unfortunately, they still occupy 4
> > bytes in each bp_cpuinfo and bp_busy_slots, as well as computing the max
> > flexible count in fetch_bp_busy_slots().
> >
> > This again causes suboptimal code generation, when we always know that
> > `!!slots.flexible` will be 0.
> >
> > Just get rid of the flexible "placeholder" and remove all real code
> > related to it. Make a note in the comment related to the constraints
> > algorithm but don't remove them from the algorithm, so that if in future
> > flexible breakpoints need supporting, it should be trivial to revive
> > them (along with reverting this change).
> >
> > Signed-off-by: Marco Elver <elver@...gle.com>
>
> Was added in 2009.
>
> Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
>
> > ---
> >  kernel/events/hw_breakpoint.c | 12 +++---------
> >  1 file changed, 3 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> > index 5f40c8dfa042..afe0a6007e96 100644
> > --- a/kernel/events/hw_breakpoint.c
> > +++ b/kernel/events/hw_breakpoint.c
> > @@ -46,8 +46,6 @@ struct bp_cpuinfo {
> >  #else
> >         unsigned int    *tsk_pinned;
> >  #endif
> > -       /* Number of non-pinned cpu/task breakpoints in a cpu */
> > -       unsigned int    flexible; /* XXX: placeholder, see fetch_this_slot() */
> >  };
> >
> >  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
> > @@ -71,7 +69,6 @@ static bool constraints_initialized __ro_after_init;
> >  /* Gather the number of total pinned and un-pinned bp in a cpuset */
> >  struct bp_busy_slots {

Do we also want to remove this struct altogether? Now it becomes just
an int counter.

> >         unsigned int pinned;
> > -       unsigned int flexible;
> >  };
> >
> >  /* Serialize accesses to the above constraints */
> > @@ -213,10 +210,6 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> >
> >                 if (nr > slots->pinned)
> >                         slots->pinned = nr;
> > -
> > -               nr = info->flexible;
> > -               if (nr > slots->flexible)
> > -                       slots->flexible = nr;
> >         }
> >  }
> >
> > @@ -299,7 +292,8 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
> >  }
> >
> >  /*
> > - * Constraints to check before allowing this new breakpoint counter:
> > + * Constraints to check before allowing this new breakpoint counter. Note that
> > + * flexible breakpoints are currently unsupported -- see fetch_this_slot().
> >   *
> >   *  == Non-pinned counter == (Considered as pinned for now)
> >   *
> > @@ -366,7 +360,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
> >         fetch_this_slot(&slots, weight);
> >
> >         /* Flexible counters need to keep at least one slot */
> > -       if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
> > +       if (slots.pinned > hw_breakpoint_slots_cached(type))
> >                 return -ENOSPC;
> >
> >         ret = arch_reserve_bp_slot(bp);
> > --
> > 2.36.1.255.ge46751e96f-goog
> >