On Tue, 24 Oct 2023 09:57:17 +0800
"wuqiang.matt" <wuqiang.m...@bytedance.com> wrote:
> On 2023/10/24 09:01, Masami Hiramatsu (Google) wrote:
> > On Mon, 23 Oct 2023 11:43:04 -0400
> > Steven Rostedt <rost...@goodmis.org> wrote:
> >
> >> On Mon, 23 Oct 2023 19:24:52 +0800
> >> "wuqiang.matt" <wuqiang.m...@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.m...@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) <mhira...@kernel.org>
