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

["wuqiang.matt" <wuqiang.matt@bytedance.com>]

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:


/* 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.

> Thank you,
> 
>>
>> -- Steve
> 

Thanks for your time,
wuqiang