Re: [PATCH net-next v2 2/2] ptr_ring: make __ptr_ring_empty() checking more reliable

[Yunsheng Lin]

On 2021/6/25 14:32, Michael S. Tsirkin wrote:
> On Fri, Jun 25, 2021 at 11:18:56AM +0800, Yunsheng Lin wrote:
>> Currently r->queue[] is cleared after r->consumer_head is moved
>> forward, which makes the __ptr_ring_empty() checking called in
>> page_pool_refill_alloc_cache() unreliable if the checking is done
>> after the r->queue clearing and before the consumer_head moving
>> forward.
> 
> 
> Well the documentation for __ptr_ring_empty clearly states is
> is not guaranteed to be reliable.

Yes, this patch does not make __ptr_ring_empty() strictly reliable
without taking the r->consumer_lock, as the disscuission in [1].

1. https://patchwork.kernel.org/project/netdevbpf/patch/1622032173-11883-1-git-send-email-linyunsheng@xxxxxxxxxx/#24207011

> 
>  *
>  * NB: This is only safe to call if ring is never resized.
>  *
>  * However, if some other CPU consumes ring entries at the same time, the value
>  * returned is not guaranteed to be correct.
>  *
>  * In this case - to avoid incorrectly detecting the ring
>  * as empty - the CPU consuming the ring entries is responsible
>  * for either consuming all ring entries until the ring is empty,
>  * or synchronizing with some other CPU and causing it to
>  * re-test __ptr_ring_empty and/or consume the ring enteries
>  * after the synchronization point.
>  *
> 
> Is it then the case that page_pool_refill_alloc_cache violates
> this requirement? How?

As my understanding:
page_pool_refill_alloc_cache() uses __ptr_ring_empty() to avoid
taking r->consumer_lock, when the above data race happens, it will
exit out and allocate page from the page allocator instead of reusing
the page in ptr_ring, which *may* not be happening if __ptr_ring_empty()
is more reliable.

> 
> It looks like you are trying to make the guarantee stronger and ensure
> no false positives.
> 
> If yes please document this as such, update the comment so all
> code can be evaluated with the eye towards whether the new stronger
> guarantee is maintained. In particular I think I see at least one
> issue with this immediately.
> 
> 
>> Move the r->queue[] clearing after consumer_head moving forward
>> to make __ptr_ring_empty() checking more reliable.
>>
>> As a side effect of above change, a consumer_head checking is
>> avoided for the likely case, and it has noticeable performance
>> improvement when it is tested using the ptr_ring_test selftest
>> added in the previous patch.
>>
>> Using "taskset -c 1 ./ptr_ring_test -s 1000 -m 0 -N 100000000"
>> to test the case of single thread doing both the enqueuing and
>> dequeuing:
>>
>>  arch     unpatched           patched       delta
>> arm64      4648 ms            4464 ms       +3.9%
>>  X86       2562 ms            2401 ms       +6.2%
>>
>> Using "taskset -c 1-2 ./ptr_ring_test -s 1000 -m 1 -N 100000000"
>> to test the case of one thread doing enqueuing and another thread
>> doing dequeuing concurrently, also known as single-producer/single-
>> consumer:
>>
>>  arch      unpatched             patched         delta
>> arm64   3624 ms + 3624 ms   3462 ms + 3462 ms    +4.4%
>>  x86    2758 ms + 2758 ms   2547 ms + 2547 ms    +7.6%
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> ---
>> V2: Add performance data.
>> ---
>>  include/linux/ptr_ring.h | 25 ++++++++++++++++---------
>>  1 file changed, 16 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>> index 808f9d3..db9c282 100644
>> --- a/include/linux/ptr_ring.h
>> +++ b/include/linux/ptr_ring.h
>> @@ -261,8 +261,7 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>>  	/* Note: we must keep consumer_head valid at all times for __ptr_ring_empty
>>  	 * to work correctly.
>>  	 */
>> -	int consumer_head = r->consumer_head;
>> -	int head = consumer_head++;
>> +	int consumer_head = r->consumer_head + 1;
>>  
>>  	/* Once we have processed enough entries invalidate them in
>>  	 * the ring all at once so producer can reuse their space in the ring.
>> @@ -271,19 +270,27 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>>  	 */
>>  	if (unlikely(consumer_head - r->consumer_tail >= r->batch ||
>>  		     consumer_head >= r->size)) {
>> +		int tail = r->consumer_tail;
>> +
>> +		if (unlikely(consumer_head >= r->size)) {
>> +			r->consumer_tail = 0;
>> +			WRITE_ONCE(r->consumer_head, 0);
>> +		} else {
>> +			r->consumer_tail = consumer_head;
>> +			WRITE_ONCE(r->consumer_head, consumer_head);
>> +		}
>> +
>>  		/* Zero out entries in the reverse order: this way we touch the
>>  		 * cache line that producer might currently be reading the last;
>>  		 * producer won't make progress and touch other cache lines
>>  		 * besides the first one until we write out all entries.
>>  		 */
>> -		while (likely(head >= r->consumer_tail))
>> -			r->queue[head--] = NULL;
>> -		r->consumer_tail = consumer_head;
>> -	}
>> -	if (unlikely(consumer_head >= r->size)) {
>> -		consumer_head = 0;
>> -		r->consumer_tail = 0;
>> +		while (likely(--consumer_head >= tail))
>> +			r->queue[consumer_head] = NULL;
>> +
>> +		return;
> 
> 
> So if now we need this to be reliable then
> we also need smp_wmb before writing r->queue[consumer_head],
> there could be other gotchas.

Yes, This patch does not make it strictly reliable.
T think I could mention that in the commit log?

> 
>>  	}
>> +
>>  	/* matching READ_ONCE in __ptr_ring_empty for lockless tests */
>>  	WRITE_ONCE(r->consumer_head, consumer_head);
>>  }
>> -- 
>> 2.7.4
> 
> 
> .
>