Re: [PATCH v1] lib,kprobes: using try_cmpxchg_local in objpool_push

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

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>