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[79.18.148.79]) by smtp.gmail.com with ESMTPSA id ga20sm1435553ejc.18.2021.08.05.01.31.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Aug 2021 01:31:47 -0700 (PDT) Date: Thu, 5 Aug 2021 10:31:45 +0200 From: Stefano Garzarella To: Kevin Wolf Subject: Re: [PATCH v2] block/io_uring: resubmit when result is -EAGAIN Message-ID: <20210805083145.idxfgmwi7i7whzjx@steredhat> References: <20210729091029.65369-1-f.ebner@proxmox.com> <20210804145048.awmlthlwlv3vcohu@steredhat> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=sgarzare@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=sgarzare@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "open list:Linux io_uring" , Stefan Hajnoczi , Julia Suvorova , qemu-devel@nongnu.org, Max Reitz , Stefan Hajnoczi , Fabian Ebner , Aarushi Mehta Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Aug 04, 2021 at 06:52:15PM +0200, Kevin Wolf wrote: >Am 04.08.2021 um 16:50 hat Stefano Garzarella geschrieben: >> On Mon, Aug 02, 2021 at 02:40:36PM +0200, Kevin Wolf wrote: >> > Am 29.07.2021 um 11:10 hat Fabian Ebner geschrieben: >> > > Linux SCSI can throw spurious -EAGAIN in some corner cases in its >> > > completion path, which will end up being the result in the completed >> > > io_uring request. >> > > >> > > Resubmitting such requests should allow block jobs to complete, even >> > > if such spurious errors are encountered. >> > > >> > > Co-authored-by: Stefan Hajnoczi >> > > Reviewed-by: Stefano Garzarella >> > > Signed-off-by: Fabian Ebner >> > > --- >> > > >> > > Changes from v1: >> > > * Focus on what's relevant for the patch itself in the commit >> > > message. >> > > * Add Stefan's comment. >> > > * Add Stefano's R-b tag (I hope that's fine, since there was no >> > > change code-wise). >> > > >> > > block/io_uring.c | 16 +++++++++++++++- >> > > 1 file changed, 15 insertions(+), 1 deletion(-) >> > > >> > > diff --git a/block/io_uring.c b/block/io_uring.c >> > > index 00a3ee9fb8..dfa475cc87 100644 >> > > --- a/block/io_uring.c >> > > +++ b/block/io_uring.c >> > > @@ -165,7 +165,21 @@ static void luring_process_completions(LuringState *s) >> > > total_bytes = ret + luringcb->total_read; >> > > >> > > if (ret < 0) { >> > > - if (ret == -EINTR) { >> > > + /* >> > > + * Only writev/readv/fsync requests on regular files or host block >> > > + * devices are submitted. Therefore -EAGAIN is not expected but it's >> > > + * known to happen sometimes with Linux SCSI. Submit again and hope >> > > + * the request completes successfully. >> > > + * >> > > + * For more information, see: >> > > + * https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u >> > > + * >> > > + * If the code is changed to submit other types of requests in the >> > > + * future, then this workaround may need to be extended to deal with >> > > + * genuine -EAGAIN results that should not be resubmitted >> > > + * immediately. >> > > + */ >> > > + if (ret == -EINTR || ret == -EAGAIN) { >> > > luring_resubmit(s, luringcb); >> > > continue; >> > > } >> > >> > Reviewed-by: Kevin Wolf >> > >> > Question about the preexisting code, though: luring_resubmit() requires >> > that the caller calls ioq_submit() later so that the request doesn't >> > just sit in a queue without getting any attention, but actually gets >> > submitted to the kernel. >> > >> > In the call chain ioq_submit() -> luring_process_completions() -> >> > luring_resubmit(), who takes care of that? >> >> Mmm, good point. >> There should be the same problem with ioq_submit() -> >> luring_process_completions() -> luring_resubmit_short_read() -> >> luring_resubmit(). >> >> Should we schedule a BH for example in luring_resubmit() to make sure that >> ioq_submit() is invoked after a resubmission? > >Or just loop in ioq_submit() after calling luring_process_completions() >if new requests were added to the queue? > I was just concerned that we might cycle a bit if a request always returns -EAGAIN, while scheduling a task might give room for other devices to queue other requests. But maybe this happens so occasionally that we might not worry about it... Stefano