Messages in this thread | | | Date | Tue, 24 Oct 2023 20:03:52 +0900 | From | Masami Hiramatsu (Google) <> | Subject | Re: [PATCH v1] lib,kprobes: using try_cmpxchg_local in objpool_push |
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On Tue, 24 Oct 2023 09:57:17 +0800 "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:
> On 2023/10/24 09:01, Masami Hiramatsu (Google) wrote: > > On Mon, 23 Oct 2023 11:43:04 -0400 > > Steven Rostedt <rostedt@goodmis.org> wrote: > > > >> On Mon, 23 Oct 2023 19:24:52 +0800 > >> "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote: > >> > >>> The objpool_push can only happen on local cpu node, so only the local > >>> cpu can touch slot->tail and slot->last, which ensures the correctness > >>> of using cmpxchg without lock prefix (using try_cmpxchg_local instead > >>> of try_cmpxchg_acquire). > >>> > >>> Testing with IACA found the lock version of pop/push pair costs 16.46 > >>> cycles and local-push version costs 15.63 cycles. Kretprobe throughput > >>> is improved to 1.019 times of the lock version for x86_64 systems. > >>> > >>> OS: Debian 10 X86_64, Linux 6.6rc6 with freelist > >>> HW: XEON 8336C x 2, 64 cores/128 threads, DDR4 3200MT/s > >>> > >>> 1T 2T 4T 8T 16T > >>> lock: 29909085 59865637 119692073 239750369 478005250 > >>> local: 30297523 60532376 121147338 242598499 484620355 > >>> 32T 48T 64T 96T 128T > >>> lock: 957553042 1435814086 1680872925 2043126796 2165424198 > >>> local: 968526317 1454991286 1861053557 2059530343 2171732306 > >>> > >>> Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com> > >>> --- > >>> lib/objpool.c | 2 +- > >>> 1 file changed, 1 insertion(+), 1 deletion(-) > >>> > >>> diff --git a/lib/objpool.c b/lib/objpool.c > >>> index ce0087f64400..a032701beccb 100644 > >>> --- a/lib/objpool.c > >>> +++ b/lib/objpool.c > >>> @@ -166,7 +166,7 @@ objpool_try_add_slot(void *obj, struct objpool_head *pool, int cpu) > >>> head = READ_ONCE(slot->head); > >>> /* fault caught: something must be wrong */ > >>> WARN_ON_ONCE(tail - head > pool->nr_objs); > >>> - } while (!try_cmpxchg_acquire(&slot->tail, &tail, tail + 1)); > >>> + } while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1)); > >>> > >>> /* now the tail position is reserved for the given obj */ > >>> WRITE_ONCE(slot->entries[tail & slot->mask], obj); > >> > >> I'm good with the change, but I don't like how "cpu" is passed to this > >> function. It currently is only used in one location, which does: > >> > >> rc = objpool_try_add_slot(obj, pool, raw_smp_processor_id()); > >> > >> Which makes this change fine. But there's nothing here to prevent someone > >> for some reason passing another CPU to that function. > >> > >> If we are to make that change, I would be much more comfortable with > >> removing "int cpu" as a parameter to objpool_try_add_slot() and adding: > >> > >> int cpu = raw_smp_processor_id(); > >> > >> Which now shows that this function *only* deals with the current CPU. > > > > Oh indeed. It used to search all CPUs to push the object, but > > I asked him to stop that because there should be enough space to > > push it in the local ring. This is a remnant of that time. > > Yes, good catch. Thanks for the explanation. > > > Wuqiang, can you make another patch to fix it? > > I'm thinking of removing the inline function objpool_try_add_slot and merging > its functionality to objpool_push, like the followings:
Looks good.
> > > /* reclaim an object to object pool */ > int objpool_push(void *obj, struct objpool_head *pool) > { > struct objpool_slot *slot; > uint32_t head, tail; > unsigned long flags; > > /* disable local irq to avoid preemption & interruption */ > raw_local_irq_save(flags); > > slot = pool->cpu_slots[raw_smp_processor_id()]; > > /* loading tail and head as a local snapshot, tail first */ > tail = READ_ONCE(slot->tail); > > do { > head = READ_ONCE(slot->head); > /* fault caught: something must be wrong */ > WARN_ON_ONCE(tail - head > pool->nr_objs); > } while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1)); > > /* now the tail position is reserved for the given obj */ > WRITE_ONCE(slot->entries[tail & slot->mask], obj); > /* update sequence to make this obj available for pop() */ > smp_store_release(&slot->last, tail + 1); > > raw_local_irq_restore(flags); > > return 0; > } > > I'll prepare a new patch for this improvement.
Thanks!
> > > Thank you, > > > >> > >> -- Steve > > > > Thanks for your time, > wuqiang
-- Masami Hiramatsu (Google) <mhiramat@kernel.org>
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