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Mon, 26 Feb 2024 22:52:22 +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 tTUiHiYW3WW+aQAAD6G6ig (envelope-from ); Mon, 26 Feb 2024 22:52:22 +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> Date: Mon, 26 Feb 2024 19:52:20 -0300 Message-ID: <87y1b6alej.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Authentication-Results: smtp-out2.suse.de; dkim=pass header.d=suse.de header.s=susede2_rsa header.b=qgWn7+Qp; dkim=pass header.d=suse.de header.s=susede2_ed25519 header.b=3eENBlYz X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-8.01 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; BAYES_HAM(-3.00)[100.00%]; MIME_GOOD(-0.10)[text/plain]; DWL_DNSWL_HI(-3.50)[suse.de:dkim]; NEURAL_HAM_LONG(-1.00)[-1.000]; 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]; DKIM_TRACE(0.00)[suse.de:+]; MX_GOOD(-0.01)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim,suse.de:email]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; NEURAL_HAM_SHORT(-0.20)[-1.000]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] X-Rspamd-Queue-Id: 34A6A1FB6F 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, 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 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. > > @@ -4368,6 +4367,15 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id) > ret = ram_load_precopy(f); > } > } > + > + /* > + * Fixed-ram migration may queue load tasks to multifd threads; make > + * sure they're all done. > + */ > + if (migrate_fixed_ram() && migrate_multifd()) { > + multifd_recv_sync_main(); > + } > + > trace_ram_load_complete(ret, seq_iter); > > return ret; > > Then ram_load() always guarantees synchronous loading of pages, and > fixed-ram will completely ignore multifd flushes (then we also skip it for > the ram_save_complete() like what this patch does for the rest). > >> >> Signed-off-by: Fabiano Rosas >> --- >> migration/ram.c | 19 ++++++++++++++++--- >> 1 file changed, 16 insertions(+), 3 deletions(-) >> >> diff --git a/migration/ram.c b/migration/ram.c >> index 5932e1b8e1..c7050f6f68 100644 >> --- a/migration/ram.c >> +++ b/migration/ram.c >> @@ -1369,8 +1369,11 @@ static int find_dirty_block(RAMState *rs, PageSearchStatus *pss) >> if (ret < 0) { >> return ret; >> } >> - qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH); >> - qemu_fflush(f); >> + >> + if (!migrate_fixed_ram()) { >> + qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH); >> + qemu_fflush(f); >> + } >> } >> /* >> * If memory migration starts over, we will meet a dirtied page >> @@ -3112,7 +3115,8 @@ static int ram_save_setup(QEMUFile *f, void *opaque) >> return ret; >> } >> >> - if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) { >> + if (migrate_multifd() && !migrate_multifd_flush_after_each_section() >> + && !migrate_fixed_ram()) { >> qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH); >> } >> >> @@ -4253,6 +4257,15 @@ static int ram_load_precopy(QEMUFile *f) >> break; >> case RAM_SAVE_FLAG_EOS: >> /* normal exit */ >> + if (migrate_fixed_ram()) { >> + /* >> + * The EOS flag appears multiple times on the >> + * stream. Fixed-ram needs only one sync at the >> + * end. It will be done on the flush flag above. >> + */ >> + break; >> + } >> + >> if (migrate_multifd() && >> migrate_multifd_flush_after_each_section()) { >> multifd_recv_sync_main(); >> -- >> 2.35.3 >>