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Wed, 31 Jan 2024 18:45:33 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id ZCLcEU2VumWHTQAAD6G6ig (envelope-from ); Wed, 31 Jan 2024 18:45:33 +0000 From: Fabiano Rosas To: peterx@redhat.com, qemu-devel@nongnu.org Cc: Bryan Zhang , Prasad Pandit , peterx@redhat.com, Yuan Liu , Avihai Horon , Hao Xiang Subject: Re: [PATCH 06/14] migration/multifd: Separate SYNC request with normal jobs In-Reply-To: <20240131103111.306523-7-peterx@redhat.com> References: <20240131103111.306523-1-peterx@redhat.com> <20240131103111.306523-7-peterx@redhat.com> Date: Wed, 31 Jan 2024 15:45:31 -0300 Message-ID: <87r0hxjq9g.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Authentication-Results: smtp-out1.suse.de; none X-Spamd-Result: default: False [-3.10 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; BAYES_HAM(-3.00)[100.00%]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCPT_COUNT_SEVEN(0.00)[8]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-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-bounces+qemu-devel=archiver.kernel.org@nongnu.org peterx@redhat.com writes: > From: Peter Xu > > Multifd provide a threaded model for processing jobs. On sender side, > there can be two kinds of job: (1) a list of pages to send, or (2) a sync > request. > > The sync request is a very special kind of job. It never contains a page > array, but only a multifd packet telling the dest side to synchronize with > sent pages. > > Before this patch, both requests use the pending_job field, no matter what > the request is, it will boost pending_job, while multifd sender thread will > decrement it after it finishes one job. > > However this should be racy, because SYNC is special in that it needs to > set p->flags with MULTIFD_FLAG_SYNC, showing that this is a sync request. > Consider a sequence of operations where: > > - migration thread enqueue a job to send some pages, pending_job++ (0->1) > > - [...before the selected multifd sender thread wakes up...] > > - migration thread enqueue another job to sync, pending_job++ (1->2), > setup p->flags=MULTIFD_FLAG_SYNC > > - multifd sender thread wakes up, found pending_job==2 > - send the 1st packet with MULTIFD_FLAG_SYNC and list of pages > - send the 2nd packet with flags==0 and no pages > > This is not expected, because MULTIFD_FLAG_SYNC should hopefully be done > after all the pages are received. Meanwhile, the 2nd packet will be > completely useless, which contains zero information. > > I didn't verify above, but I think this issue is still benign in that at > least on the recv side we always receive pages before handling > MULTIFD_FLAG_SYNC. However that's not always guaranteed and just tricky. > > One other reason I want to separate it is using p->flags to communicate > between the two threads is also not clearly defined, it's very hard to read > and understand why accessing p->flags is always safe; see the current impl > of multifd_send_thread() where we tried to cache only p->flags. It doesn't > need to be that complicated. > > This patch introduces pending_sync, a separate flag just to show that the > requester needs a sync. Alongside, we remove the tricky caching of > p->flags now because after this patch p->flags should only be used by > multifd sender thread now, which will be crystal clear. So it is always > thread safe to access p->flags. > > With that, we can also safely convert the pending_job into a boolean, > because we don't support >1 pending jobs anyway. > > Signed-off-by: Peter Xu Reviewed-by: Fabiano Rosas