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

Dmitry Vyukov
2022-06-09T12:05:15Z

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
>