Re: [PATCH v2 07/13] perf/hw_breakpoint: Remove useless code related to flexible breakpoints

Dmitry Vyukov
2022-06-28T13:21:15Z

On Tue, 28 Jun 2022 at 11:59, 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>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v2:
> * Also remove struct bp_busy_slots, and simplify functions.
> ---
> kernel/events/hw_breakpoint.c | 57 +++++++++++------------------------
> 1 file changed, 17 insertions(+), 40 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index a124786e3ade..63e39dc836bd 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -45,8 +45,6 @@ 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]);
> @@ -67,12 +65,6 @@ static const struct rhashtable_params task_bps_ht_params = {
>
> 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 */
> static DEFINE_MUTEX(nr_bp_mutex);
>
> @@ -190,14 +182,14 @@ static const struct cpumask *cpumask_of_bp(struct perf_event *bp)
> }
>
> /*
> - * Report the number of pinned/un-pinned breakpoints we have in
> - * a given cpu (cpu > -1) or in all of them (cpu = -1).
> + * Returns the max pinned breakpoint slots in a given
> + * CPU (cpu > -1) or across all of them (cpu = -1).
> */
> -static void
> -fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> - enum bp_type_idx type)
> +static int
> +max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
> {
> const struct cpumask *cpumask = cpumask_of_bp(bp);
> + int pinned_slots = 0;
> int cpu;
>
> for_each_cpu(cpu, cpumask) {
> @@ -210,24 +202,10 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> else
> nr += task_bp_pinned(cpu, bp, type);
>
> - if (nr > slots->pinned)
> - slots->pinned = nr;
> -
> - nr = info->flexible;
> - if (nr > slots->flexible)
> - slots->flexible = nr;
> + pinned_slots = max(nr, pinned_slots);
> }
> -}
>
> -/*
> - * For now, continue to consider flexible as pinned, until we can
> - * ensure no flexible event can ever be scheduled before a pinned event
> - * in a same cpu.
> - */
> -static void
> -fetch_this_slot(struct bp_busy_slots *slots, int weight)
> -{
> - slots->pinned += weight;
> + return pinned_slots;
> }
>
> /*
> @@ -298,7 +276,12 @@ __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: Flexible breakpoints are currently unimplemented, but outlined in the
> + * below algorithm for completeness. The implementation treats flexible as
> + * pinned due to no guarantee that we currently always schedule flexible events
> + * before a pinned event in a same CPU.
> *
> * == Non-pinned counter == (Considered as pinned for now)
> *
> @@ -340,8 +323,8 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
> */
> static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
> {
> - struct bp_busy_slots slots = {0};
> enum bp_type_idx type;
> + int max_pinned_slots;
> int weight;
> int ret;
>
> @@ -357,15 +340,9 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
> type = find_slot_idx(bp_type);
> weight = hw_breakpoint_weight(bp);
>
> - fetch_bp_busy_slots(&slots, bp, type);
> - /*
> - * Simulate the addition of this breakpoint to the constraints
> - * and see the result.
> - */
> - fetch_this_slot(&slots, weight);
> -
> - /* Flexible counters need to keep at least one slot */
> - if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
> + /* Check if this new breakpoint can be satisfied across all CPUs. */
> + max_pinned_slots = max_bp_pinned_slots(bp, type) + weight;
> + if (max_pinned_slots > hw_breakpoint_slots_cached(type))
> return -ENOSPC;
>
> ret = arch_reserve_bp_slot(bp);
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>