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Thu, 27 May 2021 18:32:10 -0700 (PDT) Received: from wangxiaodeMacBook-Air.local ([209.132.188.80]) by smtp.gmail.com with ESMTPSA id gg10sm2870037pjb.49.2021.05.27.18.32.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 May 2021 18:32:09 -0700 (PDT) Subject: Re: [PATCH net-next] ptr_ring: make __ptr_ring_empty() checking more reliable To: Yunsheng Lin , davem@davemloft.net, kuba@kernel.org Cc: will@kernel.org, peterz@infradead.org, paulmck@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, mst@redhat.com, brouer@redhat.com References: <1622032173-11883-1-git-send-email-linyunsheng@huawei.com> <25a6b73d-06ec-fe07-b34c-10fea709e055@huawei.com> <51bc1c38-da20-1090-e3ef-1972f28adfee@redhat.com> <938bdb23-4335-845d-129e-db8af2484c27@huawei.com> <0b64f53d-e120-f90d-bf59-bb89cceea83e@redhat.com> <758d89e8-3be1-25ae-9a42-cc8703ac097b@huawei.com> From: Jason Wang Message-ID: Date: Fri, 28 May 2021 09:31:48 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 MIME-Version: 1.0 In-Reply-To: <758d89e8-3be1-25ae-9a42-cc8703ac097b@huawei.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2021/5/27 下午5:03, Yunsheng Lin 写道: > On 2021/5/27 16:05, Jason Wang wrote: >> 在 2021/5/27 下午3:21, Yunsheng Lin 写道: >>> On 2021/5/27 14:53, Jason Wang wrote: >>>> 在 2021/5/27 下午2:07, Yunsheng Lin 写道: >>>>> On 2021/5/27 12:57, Jason Wang wrote: >>>>>> 在 2021/5/26 下午8:29, Yunsheng Lin 写道: >>>>>>> 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. >>>>>>> >>>>>>> Move the r->queue[] clearing after consumer_head moving forward >>>>>>> to make __ptr_ring_empty() checking more reliable. >>>>>> If I understand this correctly, this can only happens if you run __ptr_ring_empty() in parallel with ptr_ring_discard_one(). >>>>> Yes. >>>>> >>>>>> I think those two needs to be serialized. Or did I miss anything? >>>>> As the below comment in __ptr_ring_discard_one, if the above is true, I >>>>> do not think we need to keep consumer_head valid at all times, right? >>>>> >>>>> >>>>> /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty >>>>> * to work correctly. >>>>> */ >>>> I'm not sure I understand. But my point is that you need to synchronize the __ptr_ring_discard_one() and __ptr_empty() as explained in the comment above __ptr_ring_empty(): >>> I am saying if __ptr_ring_empty() and __ptr_ring_discard_one() is >>> always serialized, then it seems that the below commit is unnecessary? >> >> Just to make sure we are at the same page. What I really meant is "synchronized" not "serialized". So they can be called at the same time but need synchronization. >> >> >>> 406de7555424 ("ptr_ring: keep consumer_head valid at all times") >> >> This still needed in this case. >> >> >>>> /* >>>> * Test ring empty status without taking any locks. >>>> * >>>> * 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. >>> I am not sure I understand "incorrectly detecting the ring as empty" >>> means, is it because of the data race described in the commit log? >> >> It means "the ring might be empty but __ptr_ring_empty() returns false". > But the ring might be non-empty but __ptr_ring_empty() returns true > for the data race described in the commit log:) Which commit log? > >> >>> Or other data race? I can not think of other data race if consuming >>> and __ptr_ring_empty() is serialized:) >>> >>> I am agreed that __ptr_ring_empty() checking is not totally reliable >>> without taking r->consumer_lock, that is why I use "more reliable" >>> in the title:) >> >> Is __ptr_ring_empty() synchronized with the consumer in your case? If yes, have you done some benchmark to see the difference? >> >> Have a look at page pool, this only helps when multiple refill request happens in parallel which can make some of the refill return early if the ring has been consumed. >> >> This is the slow-path and I'm not sure we see any difference. If one the request runs faster then the following request will go through the fast path. > Yes, I am agreed there may not be any difference. > But it is better to make it more reliable, right? No, any performance optimization must be benchmark to show obvious difference to be accepted. ptr_ring has been used by various subsystems so we should not risk our self-eves to accept theoretical optimizations. > >> If it really helps, can we do it more simpler by: >> >> >> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h >> index 808f9d3ee546..c3a72dc77337 100644 >> --- a/include/linux/ptr_ring.h >> +++ b/include/linux/ptr_ring.h >> @@ -264,6 +264,10 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) >> int consumer_head = r->consumer_head; >> int head = consumer_head++; >> >> + /* matching READ_ONCE in __ptr_ring_empty for lockless tests */ >> + WRITE_ONCE(r->consumer_head, >> + consumer_head < r->size ? consumer_head : 0); >> + >> /* Once we have processed enough entries invalidate them in >> * the ring all at once so producer can reuse their space in the ring. >> * We also do this when we reach end of the ring - not mandatory >> @@ -281,11 +285,8 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) >> r->consumer_tail = consumer_head; >> } >> if (unlikely(consumer_head >= r->size)) { > What I am thinking is that we can remove the above testing for > the likely case when the above checking is moved into the body > of "if (unlikely(consumer_head - r->consumer_tail >= r->batch || > consumer_head >= r->size))". > > Or is there any specific reason why we keep the testing for likely > case? No reason but any optimization must be tested to show differences before being accepted. Thanks > > >> - consumer_head = 0; >> r->consumer_tail = 0; >> } >> - /* matching READ_ONCE in __ptr_ring_empty for lockless tests */ >> - WRITE_ONCE(r->consumer_head, consumer_head); >> } >> >> static inline void *__ptr_ring_consume(struct ptr_ring *r) >> >> >> Thanks >> >> >>> >>> >>>> * >>>> * Note: callers invoking this in a loop must use a compiler barrier, >>>> * for example cpu_relax(). >>>> */ >>>> >>>> Thanks >>>> >>>> >>>> >> >> . >>