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[82.30.61.225]) by smtp.gmail.com with ESMTPSA id l15-20020a05600c4f0f00b003942a244f39sm22772598wmq.18.2022.06.14.07.14.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Jun 2022 07:14:28 -0700 (PDT) Date: Tue, 14 Jun 2022 15:14:26 +0100 From: "Dr. David Alan Gilbert" To: chuang xu Cc: Leonardo Bras , qemu-devel@nongnu.org, Peter Xu , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Paolo Bonzini , Elena Ufimtseva , Jagannathan Raman , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , John G Johnson , Juan Quintela , Eric Blake , Markus Armbruster , Fam Zheng , lizefan.x@bytedance.com, zhouyibo@bytedance.com Subject: Re: [External] [PATCH v13 3/8] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX Message-ID: References: <20220513062836.965425-1-leobras@redhat.com> <20220513062836.965425-4-leobras@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.5 (2022-05-16) Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01, T_SPF_HELO_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * chuang xu (xuchuangxclwt@bytedance.com) wrote: > > On 2022/5/13 下午2:28, Leonardo Bras wrote: > > @@ -557,15 +578,31 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc, > > memcpy(CMSG_DATA(cmsg), fds, fdsize); > > } > > +#ifdef QEMU_MSG_ZEROCOPY > > + if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) { > > + sflags = MSG_ZEROCOPY; > > + } > > +#endif > > + > > retry: > > - ret = sendmsg(sioc->fd, &msg, 0); > > + ret = sendmsg(sioc->fd, &msg, sflags); > > if (ret <= 0) { > > - if (errno == EAGAIN) { > > + switch (errno) { > > + case EAGAIN: > > return QIO_CHANNEL_ERR_BLOCK; > > - } > > - if (errno == EINTR) { > > + case EINTR: > > goto retry; > > +#ifdef QEMU_MSG_ZEROCOPY > > + case ENOBUFS: > > + if (sflags & MSG_ZEROCOPY) { > > + error_setg_errno(errp, errno, > > + "Process can't lock enough memory for using MSG_ZEROCOPY"); > > + return -1; > > + } > > + break; > > +#endif > > } > > + > > error_setg_errno(errp, errno, > > "Unable to write to socket"); > > return -1; > > Hi, Leo. > > There are some other questions I would like to discuss with you. > > I tested the multifd zero_copy migration and found that sometimes even if > max locked memory of qemu was set to 16GB（much greater than > `MULTIFD_PACKET_SIZE`）, the error "Process can't lock enough memory for > using MSG_ZEROCOPY" would still be reported. > > I noticed that the > doc(https://www.kernel.org/doc/html/v5.12/networking/msg_zerocopy.html) says > "A zerocopy failure will return -1 with errno ENOBUFS. This happens if the > socket option was not set, _the socket exceeds its optmem limit_ or the user > exceeds its ulimit on locked pages." > > I also found that the RFC(https://lwn.net/Articles/715279/) says _"__The > change to allocate notification skbuffs from optmem requires__ensuring that > net.core.optmem is at least a few 100KB."_ Interesting. > On my host,  optmem was initially set to 20KB, I tried to change it to 100KB > (echo 102400 > /proc/sys/net/core/optmem_max) as the RFC says.Then I tested > the multifd zero_copy migration repeatedly，and the error disappeared. > > So when sendmsg returns -1 with errno ENOBUFS, should we distinguish between > error ''socket exceeds optmem limit" and error "user exceeds ulimit on > locked pages"? Or is there any better way to avoid this problem? I don't think we can tell which one of them triggered the error; so the only thing I can suggest is that we document the need for optmem_max setting; I wonder how we get a better answer than 'a few 100KB'? I guess it's something like the number of packets inflight * sizeof(cmsghdr) ? Dave > Best Regards, > > chuang xu -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK