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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 9 Jun 2022 14:04:55 +0200
Message-ID: <CACT4Y+Zd0Zd_66DZ-f2HG4tR6ZdraFe9b4iEBJmG9p72+7RMWQ@mail.gmail.com>
Subject: Re: [PATCH 5/8] perf/hw_breakpoint: Remove useless code related to
 flexible breakpoints
To:     Marco Elver <elver@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, x86@kernel.org,
        linux-sh@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-kernel@vger.kernel.org
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On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@google.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@google.com>

Was added in 2009.

Acked-by: Dmitry Vyukov <dvyukov@google.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 {
>         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
>