Messages in this thread | | | From | Dmitry Vyukov <> | Date | Tue, 28 Jun 2022 15:18:49 +0200 | Subject | Re: [PATCH v2 07/13] perf/hw_breakpoint: Remove useless code related to flexible breakpoints |
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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 >
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