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Thu, 27 May 2021 19:29:32 -0700 (PDT) Received: from wangxiaodeMacBook-Air.local ([209.132.188.80]) by smtp.gmail.com with ESMTPSA id w11sm2907612pfc.79.2021.05.27.19.29.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 May 2021 19:29:32 -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: <97021854-739a-682c-f2b6-d609dfcfa971@redhat.com> Date: Fri, 28 May 2021 10:29:27 +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: 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 2021/5/28 上午10:26, Yunsheng Lin 写道: > On 2021/5/28 9:31, Jason Wang wrote: >> 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? > this commit log. > If the data race described in this commit log happens, the ring might be > non-empty, but __ptr_ring_empty() returns true. > > >> >>>>> 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. > As a matter of fact, I am not treating it as a performance optimization for this patch. > I treated it as improvement for the checking of __ptr_ring_empty(). > But you are right that we need to ensure there is not performance regression when improving > it. > > Any existing and easy-to-setup testcase to benchmark the ptr_ring performance? You probably can start with a simple test in: tools/virtio/ringtest/ptr_ring.c Thanks > >> >>>> If it really helps, can we do it more simpler by: >>>> >