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David Alan Gilbert" , peterx@redhat.com, Alexey Perevalov , Juraj Marcin Subject: Re: [PATCH 02/13] migration/postcopy: Push blocktime start/end into page req mutex In-Reply-To: <20250527231248.1279174-3-peterx@redhat.com> References: <20250527231248.1279174-1-peterx@redhat.com> <20250527231248.1279174-3-peterx@redhat.com> Date: Mon, 02 Jun 2025 14:46:09 -0300 Message-ID: <87seki3ula.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_FIVE(0.00)[6]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo, suse.de:mid, suse.de:email] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.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 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 Peter Xu writes: > The postcopy blocktime feature was tricky that it used quite some atomic > operations over quite a few arrays and vars, without explaining how that > would be thread safe. The thread safety here is about concurrency between > the fault thread and the fault resolution threads, possible to access the > same chunk of data. All these atomic ops can be expensive too before > knowing clearly how it works. > > OTOH, postcopy has one page_request_mutex used to serialize the received > bitmap updates. So far it's ok - we don't yet have a lot of threads > contending the lock. It might change after multifd will be supported, but > that's a separate story. What is important is, with that mutex, it's > pretty lightweight to move all the blocktime maintenance into the mutex > critical section. It's because the blocktime layer is lightweighted: > almost "remember which vcpu faulted on which address", and "ok we get some > fault resolved, calculate how long it takes". It's also an optional > feature for now (but I have thought of changing that, maybe in the future). > > Let's push the blocktime layer into the mutex, so that it's always > thread-safe even without any atomic ops. > > To achieve that, I'll need to add a tid parameter on fault path so that > it'll start to pass the faulted thread ID into deeper the stack, but not > too deep. When at it, add a comment for the shared fault handler (for > example, vhost-user devices running with postcopy), to mention a TODO. One > reason it might not be trivial is that vhost-user's userfaultfds should be > opened by vhost-user process, so it's pretty hard to control making sure > the TID feature will be around. It wasn't supported before, so keep it > like that for now. > > Now we should be as ease when everything is protected by a mutex that we > always take anyway. > > One side effect: we can finally remove one ramblock_recv_bitmap_test() in > mark_postcopy_blocktime_begin(), which was pretty weird and which also > includes a weird (but maybe necessary.. but maybe not?) operation to inject > a blocktime entry then quickly erase it.. When we're with the mutex, and > when we make sure it's invoked after checking the receive bitmap, it's not > needed anymore. Instead, we assert. > > As another side effect, this paves way for removing all atomic ops in all > the mem accesses in blocktime layer. > > Note that we need a stub for mark_postcopy_blocktime_begin() for Windows > builds. > > Signed-off-by: Peter Xu Reviewed-by: Fabiano Rosas