Re: [PATCH 3/8] perf/hw_breakpoint: Optimize constant number of breakpoint slots

Dmitry Vyukov
2022-06-09T11:56:14Z

On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@google.com> wrote:
>
> Optimize internal hw_breakpoint state if the architecture's number of
> breakpoint slots is constant. This avoids several kmalloc() calls and
> potentially unnecessary failures if the allocations fail, as well as
> subtly improves code generation and cache locality.
>
> The protocol is that if an architecture defines hw_breakpoint_slots via
> the preprocessor, it must be constant and the same for all types.
>
> Signed-off-by: Marco Elver <elver@google.com>

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

> ---
> arch/sh/include/asm/hw_breakpoint.h | 5 +-
> arch/x86/include/asm/hw_breakpoint.h | 5 +-
> kernel/events/hw_breakpoint.c | 92 ++++++++++++++++++----------
> 3 files changed, 62 insertions(+), 40 deletions(-)
>
> diff --git a/arch/sh/include/asm/hw_breakpoint.h b/arch/sh/include/asm/hw_breakpoint.h
> index 199d17b765f2..361a0f57bdeb 100644
> --- a/arch/sh/include/asm/hw_breakpoint.h
> +++ b/arch/sh/include/asm/hw_breakpoint.h
> @@ -48,10 +48,7 @@ struct pmu;
> /* Maximum number of UBC channels */
> #define HBP_NUM 2
>
> -static inline int hw_breakpoint_slots(int type)
> -{
> - return HBP_NUM;
> -}
> +#define hw_breakpoint_slots(type) (HBP_NUM)
>
> /* arch/sh/kernel/hw_breakpoint.c */
> extern int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw);
> diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
> index a1f0e90d0818..0bc931cd0698 100644
> --- a/arch/x86/include/asm/hw_breakpoint.h
> +++ b/arch/x86/include/asm/hw_breakpoint.h
> @@ -44,10 +44,7 @@ struct arch_hw_breakpoint {
> /* Total number of available HW breakpoint registers */
> #define HBP_NUM 4
>
> -static inline int hw_breakpoint_slots(int type)
> -{
> - return HBP_NUM;
> -}
> +#define hw_breakpoint_slots(type) (HBP_NUM)
>
> struct perf_event_attr;
> struct perf_event;
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 1f718745d569..8e939723f27d 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -41,13 +41,16 @@ struct bp_cpuinfo {
> /* Number of pinned cpu breakpoints in a cpu */
> unsigned int cpu_pinned;
> /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
> +#ifdef hw_breakpoint_slots
> + unsigned int tsk_pinned[hw_breakpoint_slots(0)];
> +#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]);
> -static int nr_slots[TYPE_MAX] __ro_after_init;
>
> static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
> {
> @@ -74,6 +77,54 @@ struct bp_busy_slots {
> /* Serialize accesses to the above constraints */
> static DEFINE_MUTEX(nr_bp_mutex);
>
> +#ifdef hw_breakpoint_slots
> +/*
> + * Number of breakpoint slots is constant, and the same for all types.
> + */
> +static_assert(hw_breakpoint_slots(TYPE_INST) == hw_breakpoint_slots(TYPE_DATA));
> +static inline int hw_breakpoint_slots_cached(int type) { return hw_breakpoint_slots(type); }
> +static inline int init_breakpoint_slots(void) { return 0; }
> +#else
> +/*
> + * Dynamic number of breakpoint slots.
> + */
> +static int __nr_bp_slots[TYPE_MAX] __ro_after_init;
> +
> +static inline int hw_breakpoint_slots_cached(int type)
> +{
> + return __nr_bp_slots[type];
> +}
> +
> +static __init int init_breakpoint_slots(void)
> +{
> + int i, cpu, err_cpu;
> +
> + for (i = 0; i < TYPE_MAX; i++)
> + __nr_bp_slots[i] = hw_breakpoint_slots(i);
> +
> + for_each_possible_cpu(cpu) {
> + for (i = 0; i < TYPE_MAX; i++) {
> + struct bp_cpuinfo *info = get_bp_info(cpu, i);
> +
> + info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
> + if (!info->tsk_pinned)
> + goto err;
> + }
> + }
> +
> + return 0;
> +err:
> + for_each_possible_cpu(err_cpu) {
> + for (i = 0; i < TYPE_MAX; i++)
> + kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> + if (err_cpu == cpu)
> + break;
> + }
> +
> + return -ENOMEM;
> +}
> +#endif
> +
> __weak int hw_breakpoint_weight(struct perf_event *bp)
> {
> return 1;
> @@ -96,7 +147,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
> unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> int i;
>
> - for (i = nr_slots[type] - 1; i >= 0; i--) {
> + for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> if (tsk_pinned[i] > 0)
> return i + 1;
> }
> @@ -313,7 +364,7 @@ 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) > nr_slots[type])
> + if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
> return -ENOSPC;
>
> ret = arch_reserve_bp_slot(bp);
> @@ -688,42 +739,19 @@ static struct pmu perf_breakpoint = {
>
> int __init init_hw_breakpoint(void)
> {
> - int cpu, err_cpu;
> - int i, ret;
> -
> - for (i = 0; i < TYPE_MAX; i++)
> - nr_slots[i] = hw_breakpoint_slots(i);
> -
> - for_each_possible_cpu(cpu) {
> - for (i = 0; i < TYPE_MAX; i++) {
> - struct bp_cpuinfo *info = get_bp_info(cpu, i);
> -
> - info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
> - GFP_KERNEL);
> - if (!info->tsk_pinned) {
> - ret = -ENOMEM;
> - goto err;
> - }
> - }
> - }
> + int ret;
>
> ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
> if (ret)
> - goto err;
> + return ret;
> +
> + ret = init_breakpoint_slots();
> + if (ret)
> + return ret;
>
> constraints_initialized = true;
>
> perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
>
> return register_die_notifier(&hw_breakpoint_exceptions_nb);
> -
> -err:
> - for_each_possible_cpu(err_cpu) {
> - for (i = 0; i < TYPE_MAX; i++)
> - kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> - if (err_cpu == cpu)
> - break;
> - }
> -
> - return ret;
> }
> --
> 2.36.1.255.ge46751e96f-goog
>