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Tue, 27 Feb 2024 14:00:46 +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 fcdbMg7r3WVtNwAAD6G6ig (envelope-from ); Tue, 27 Feb 2024 14:00:46 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, berrange@redhat.com, armbru@redhat.com, Claudio Fontana Subject: Re: [PATCH v4 22/34] migration/multifd: Prepare multifd sync for fixed-ram migration In-Reply-To: References: <20240220224138.24759-1-farosas@suse.de> <20240220224138.24759-23-farosas@suse.de> <87y1b6alej.fsf@suse.de> Date: Tue, 27 Feb 2024 11:00:44 -0300 Message-ID: <87plwivwfn.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Authentication-Results: smtp-out2.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]; RCPT_COUNT_FIVE(0.00)[5]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; 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=195.135.223.131; 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, 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 Peter Xu writes: > On Mon, Feb 26, 2024 at 07:52:20PM -0300, Fabiano Rosas wrote: >> Peter Xu writes: >> >> > On Tue, Feb 20, 2024 at 07:41:26PM -0300, Fabiano Rosas wrote: >> >> The fixed-ram migration can be performed live or non-live, but it is >> >> always asynchronous, i.e. the source machine and the destination >> >> machine are not migrating at the same time. We only need some pieces >> >> of the multifd sync operations. >> >> >> >> multifd_send_sync_main() >> >> ------------------------ >> >> Issued by the ram migration code on the migration thread, causes the >> >> multifd send channels to synchronize with the migration thread and >> >> makes the sending side emit a packet with the MULTIFD_FLUSH flag. >> >> >> >> With fixed-ram we want to maintain the sync on the sending side >> >> because that provides ordering between the rounds of dirty pages when >> >> migrating live. >> >> >> >> MULTIFD_FLUSH >> >> ------------- >> >> On the receiving side, the presence of the MULTIFD_FLUSH flag on a >> >> packet causes the receiving channels to start synchronizing with the >> >> main thread. >> >> >> >> We're not using packets with fixed-ram, so there's no MULTIFD_FLUSH >> >> flag and therefore no channel sync on the receiving side. >> >> >> >> multifd_recv_sync_main() >> >> ------------------------ >> >> Issued by the migration thread when the ram migration flag >> >> RAM_SAVE_FLAG_MULTIFD_FLUSH is received, causes the migration thread >> >> on the receiving side to start synchronizing with the recv >> >> channels. Due to compatibility, this is also issued when >> >> RAM_SAVE_FLAG_EOS is received. >> >> >> >> For fixed-ram we only need to synchronize the channels at the end of >> >> migration to avoid doing cleanup before the channels have finished >> >> their IO. >> >> >> >> Make sure the multifd syncs are only issued at the appropriate >> >> times. Note that due to pre-existing backward compatibility issues, we >> >> have the multifd_flush_after_each_section property that enables an >> >> older behavior of synchronizing channels more frequently (and >> >> inefficiently). Fixed-ram should always run with that property >> >> disabled (default). >> > >> > What if the user enables multifd_flush_after_each_section=true? >> > >> > IMHO we don't necessarily need to attach the fixed-ram loading flush to any >> > flag in the stream. For fixed-ram IIUC all the loads will happen in one >> > shot of ram_load() anyway when parsing the ramblock list, so.. how about we >> > decouple the fixed-ram load flush from the stream by always do a sync in >> > ram_load() unconditionally? >> >> I would like to. But it's not possible because ram_load() is called once >> per section. So once for each EOS flag on the stream. We'll have at >> least two calls to ram_load(), once due to qemu_savevm_state_iterate() >> and another due to qemu_savevm_state_complete_precopy(). >> >> The fact that fixed-ram can use just one load doesn't change the fact >> that we perform more than one "save". So we'll need to use the FLUSH >> flag in this case unfortunately. > > After I re-read it, I found one more issue. > > Now recv side sync is "once and for all" - it doesn't allow a second time > to sync_main because it syncs only until quits. That is IMHO making the > code much harder to maintain, and we'll need rich comment to explain why is > that happening. > > Ideally any "sync main" for recv threads can be called multiple times. And > IMHO it's not really hard. Actually it can make the code much cleaner by > merging some logic between socket-based and file-based from that regard. > > I tried to play with your branch and propose something like this, just to > show what I meant. This should allow all new fixed-ram test to pass here, > meanwhile it should allow sync main on recv side to be re-entrant, sharing > the logic with socket-based as much as possible: > > ===== > diff --git a/migration/multifd.c b/migration/multifd.c > index a0202b5661..28480f6cfe 100644 > --- a/migration/multifd.c > +++ b/migration/multifd.c > @@ -86,10 +86,8 @@ struct { > /* number of created threads */ > int count; > /* > - * For sockets: this is posted once for each MULTIFD_FLAG_SYNC flag. > - * > - * For files: this is only posted at the end of the file load to mark > - * completion of the load process. > + * This is always posted by the recv threads, the main thread uses it > + * to wait for recv threads to finish assigned tasks. > */ > QemuSemaphore sem_sync; > /* global number of generated multifd packets */ > @@ -1316,38 +1314,55 @@ void multifd_recv_cleanup(void) > multifd_recv_cleanup_state(); > } > > - > -/* > - * Wait until all channels have finished receiving data. Once this > - * function returns, cleanup routines are safe to run. > - */ > -static void multifd_file_recv_sync(void) > +static void multifd_recv_file_sync_request(void) > { > int i; > > for (i = 0; i < migrate_multifd_channels(); i++) { > MultiFDRecvParams *p = &multifd_recv_state->params[i]; > > - trace_multifd_recv_sync_main_wait(p->id); > - > + /* > + * We play a trick here: instead of using a separate pending_sync > + * to send a sync request (like what we do on senders), we simply > + * kick the recv thread once without setting pending_job. > + * > + * If there's already a pending_job, the thread will only see it > + * after it processed the current. If there's no pending_job, > + * it'll see this immediately. > + */ > qemu_sem_post(&p->sem); > - > trace_multifd_recv_sync_main_signal(p->id); > - qemu_sem_wait(&p->sem_sync); > } > - return; > } > > +/* > + * Request a sync for all the multifd recv threads. > + * > + * For socket-based, sync request is much more complicated, which relies on > + * collaborations between both explicit RAM_SAVE_FLAG_MULTIFD_FLUSH in the > + * main stream, and MULTIFD_FLAG_SYNC flag in per-channel protocol. Here > + * it should be invoked by the main stream request. > + * > + * For file-based, it is much simpler, because there's no need for a strong > + * sync semantics between the main thread and the recv threads. What we > + * need is only to make sure all recv threads finished their tasks. > + */ > void multifd_recv_sync_main(void) > { > + bool file_based = !multifd_use_packets(); > int i; > > if (!migrate_multifd()) { > return; > } > > - if (!multifd_use_packets()) { > - return multifd_file_recv_sync(); > + if (file_based) { > + /* > + * File-based multifd requires an explicit sync request because > + * tasks are assigned by the main recv thread, rather than parsed > + * through the multifd channels. > + */ > + multifd_recv_file_sync_request(); > } > > for (i = 0; i < migrate_multifd_channels(); i++) { > @@ -1356,6 +1371,11 @@ void multifd_recv_sync_main(void) > trace_multifd_recv_sync_main_wait(p->id); > qemu_sem_wait(&multifd_recv_state->sem_sync); > } > + > + if (file_based) { > + return; > + } > + > for (i = 0; i < migrate_multifd_channels(); i++) { > MultiFDRecvParams *p = &multifd_recv_state->params[i]; > > @@ -1420,11 +1440,12 @@ static void *multifd_recv_thread(void *opaque) > } > > /* > - * Migration thread did not send work, break and signal > - * sem_sync so it knows we're not lagging behind. > + * Migration thread did not send work, this emulates > + * pending_sync, post sem_sync to notify the main thread. > */ > if (!qatomic_read(&p->pending_job)) { > - break; > + qemu_sem_post(&multifd_recv_state->sem_sync); > + continue; > } > > has_data = !!p->data->size; > @@ -1449,10 +1470,6 @@ static void *multifd_recv_thread(void *opaque) > } > } > > - if (!use_packets) { > - qemu_sem_post(&p->sem_sync); > - } > - > if (local_err) { > multifd_recv_terminate_threads(local_err); > error_free(local_err); > > ========== > > Note that I used multifd_recv_state->sem_sync to send the message rather > than p->sem, not only because socket-based has similar logic on using that > sem, but also because main thread shouldn't care about "which" recv thread > has finished, but "all recv threads are idle". > > Do you think this should work out for us in a nicer way? > I don't really like the interleaving of file and socket logic at multifd_recv_sync_main(), but I can live with it. Waiting on multifd_recv_state->sem_sync is problematic because if the thread has an error, that will hang forever. Actually, I don't even see this being handled in _current_ code anywhere, we probably have a bug there. I guess we need to add one more "post this sem just because" somewhere. multifd_recv_kick_main probably. > Then we talk about the other issue, on whether we should rely on migration > stream to flush recv threads. My answer is still hopefully a no. > > In the ideal case, fixed-ram image format should even be tailed to not use > a live stream protocol. For example, currently during ram iterations we > should flush quite a lot of ram QEMU_VM_SECTION_PART sections contains > mostly rubbish but then ending that with RAM_SAVE_FLAG_EOS. Then we keep > doing this in the iteration loop. Here the real meat is during processing > of QEMU_VM_SECTION_PART, the src QEMU will update the guest pages with > fixed offsets in the file. That however doesn't really contribute to > anything valuable in the migration stream itself (things sent over > to_dst_file). > > AFAIU we chose to still use that logic only for simplicity, even if we know > those EOSs and all RAM streams are garbage. Now we tend to add one > dependency on part of the garbage, which is RAM_SAVE_FLAG_MULTIFD_FLUSH in > this case; which is useful in socket-based but shouldn't be necessary for > file. > > I think I have a solution besides ram_load(): ultimately fixed-ram stores > all guest mem in the QEMU_VM_SECTION_START section of the ram, through all > of the RAM_SAVE_FLAG_MEM_SIZE (which leads to parse_ramblocks()). If so, > perhaps we can do one shot sync for file at the end of parse_ramblocks()? > Then we decouple sync_main on recv for file-based completely against all > stream flags. Yeah, that could work. I think I'll blacklist all unused flags using the invalid_flags logic. Thanks