From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 88BE3E7716E for ; Thu, 5 Dec 2024 21:16:54 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tJJCt-0004Vv-8M; Thu, 05 Dec 2024 16:16:19 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tJJCr-0004VQ-57 for qemu-devel@nongnu.org; Thu, 05 Dec 2024 16:16:17 -0500 Received: from smtp-out2.suse.de ([2a07:de40:b251:101:10:150:64:2]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1tJJCp-0002sn-79 for qemu-devel@nongnu.org; Thu, 05 Dec 2024 16:16:16 -0500 Received: from imap1.dmz-prg2.suse.org (unknown [10.150.64.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 2CE8D1F37C; Thu, 5 Dec 2024 21:16:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1733433370; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=GCzzmwH+qWmJr3FqIA0xXzUGmmRBmF9JtkbfML9wI8I=; b=vWFQvMEJnyFuiZokUghrG8p83e9g2aC20xwBNMh2LuXf3lqANjeGDXYdr1S4FOJg4J50/s WbXiQoopRVsDiDt3PE7jrYOxRYJQnz9gUwV87Bn9Pjp0z7XYLNh/mDzuDZnDK0mVgHqQ70 LLaPVRpb36DDdZWxznEhY9vBZEO15uo= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1733433370; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=GCzzmwH+qWmJr3FqIA0xXzUGmmRBmF9JtkbfML9wI8I=; b=5aEVmHUUOjwGUiXvEisL3gyemO11u3NDZtbSiJ6zZ8aMhpEXTmaDc0/5VrWxTnOhzshGhE d32a8Vj5qHcLD/DA== Authentication-Results: smtp-out2.suse.de; none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1733433369; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=GCzzmwH+qWmJr3FqIA0xXzUGmmRBmF9JtkbfML9wI8I=; b=v8iqc8eM4EskpWwLk3/FhnN4wcmUyMFCsVF2mIwHBjKqihtcCI/HXca4MCH5zE9KrEC9pT 4F0HXCaY+XUSObw/Lpu79Ule81KE5Yn+r1jT3IRn2J7J18nobNUQVx9mJbU6Ou/GcBH4Bi G332F7XIabDl63m9PAqFP8Vq4PFdyMc= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1733433369; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=GCzzmwH+qWmJr3FqIA0xXzUGmmRBmF9JtkbfML9wI8I=; b=Stfi4hUTVb6oTsUgR8CN7QsxpGEzUv39VOU2L5ZCFLo4tKtir7TT0vhitWamqAsyjGmC2o BDwGQfcrw90hM8Dg== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id A3DB6132EB; Thu, 5 Dec 2024 21:16:08 +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 +4YIGhgYUmcNWgAAD6G6ig (envelope-from ); Thu, 05 Dec 2024 21:16:08 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, "Maciej S . Szmigiero" , Alex Williamson , Avihai Horon , =?utf-8?Q?C=C3=A9dric?= Le Goater Subject: Re: [PATCH 1/2] migration/multifd: Further remove the SYNC on complete In-Reply-To: References: <20241205185303.897010-1-peterx@redhat.com> <20241205185303.897010-2-peterx@redhat.com> <878qstj46b.fsf@suse.de> Date: Thu, 05 Dec 2024 18:16:05 -0300 Message-ID: <87jzcdde5m.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] 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: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, 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: > On Thu, Dec 05, 2024 at 04:55:08PM -0300, Fabiano Rosas wrote: >> Peter Xu writes: >> >> > Commit 637280aeb2 ("migration/multifd: Avoid the final FLUSH in >> > complete()") removed the FLUSH operation on complete() which should avoid >> > one global sync on destination side, because it's not needed. >> > >> > However that commit overlooked multifd_ram_flush_and_sync() part of things, >> > as that's always used together with the FLUSH message on the main channel. >> >> Let's please write the full name of the flags, functions, etc. As we've >> seen in the discussion with Prasad, we're stumbling over ambiguous >> terminology. This is RAM_SAVE_FLAG_MULTIFD_FLUSH. > > Sure. > >> >> > >> > For very old binaries (multifd_flush_after_each_section==true), that's >> > still needed because each EOS received on destination will enforce >> > all-channel sync once. >> >> Ok, but why does this patch not reinstate the flag? > > RAM_SAVE_FLAG_MULTIFD_FLUSH? Because it's not needed? > Ah, you're saying we need the source side to send the MULTIFD_FLAG_SYNC packet so that the old QEMU on the recv side gets out of waiting. I see. >> >> > >> > For new binaries (multifd_flush_after_each_section==false), the flush and >> > sync shouldn't be needed just like the FLUSH, with the same reasoning. >> > >> > Currently, on new binaries we'll still send SYNC messages on multifd >> > channels, even if FLUSH is omitted at the end. It'll make all recv threads >> > hang at SYNC message. >> >> I don't get this part, is this a bug you're describing? There's not SYNC >> message on the recv side, I think you mean the MULTIFD_FLAG_SYNC and >> this code? >> >> if (flags & MULTIFD_FLAG_SYNC) { >> qemu_sem_post(&multifd_recv_state->sem_sync); >> qemu_sem_wait(&p->sem_sync); >> } > > Yes. > >> >> That's not a hang, that's the sync. > > When recv side never collect that SYNC (by invoke multifd_recv_sync_main), > then it is a hang. > Right, I forget the sync on the recv is the other way around. It's the multifd_recv_state that does the sync between the threads. The sem_sync is there so that the channels don't exit. >> >> > >> > Multifd is still all working fine because luckily recv side cleanup >> > code (mostly multifd_recv_sync_main()) is smart enough to make sure even if >> > recv threads are stuck at SYNC it'll get kicked out. And since this is the >> > completion phase of migration, nothing else will be sent after the SYNCs. >> >> Hmm, a last sync on the recv side might indeed not be needed. >> >> > >> > This may be needed for VFIO in the future because VFIO can have data to >> > push after ram_save_complete(), hence we don't want the recv thread to be >> > stuck in SYNC message. Remove this limitation will make src even slightly >> > faster too to avoid some more code. >> > >> > Stable branches do not need this patch, as no real bug I can think of that >> > will go wrong there.. so not attaching Fixes to be clear on the backport >> > not needed. >> > >> > Signed-off-by: Peter Xu >> > --- >> > migration/ram.c | 13 ++++++++++--- >> > 1 file changed, 10 insertions(+), 3 deletions(-) >> > >> > diff --git a/migration/ram.c b/migration/ram.c >> > index 05ff9eb328..7284c34bd8 100644 >> > --- a/migration/ram.c >> > +++ b/migration/ram.c >> > @@ -3283,9 +3283,16 @@ static int ram_save_complete(QEMUFile *f, void *opaque) >> > } >> > } >> > >> > - ret = multifd_ram_flush_and_sync(); >> > - if (ret < 0) { >> > - return ret; >> > + if (migrate_multifd() && >> > + migrate_multifd_flush_after_each_section()) { >> > + /* >> > + * Only the old dest QEMU will need this sync, because each EOS >> > + * will require one SYNC message on each channel. >> > + */ >> > + ret = multifd_ram_flush_and_sync(); >> > + if (ret < 0) { >> > + return ret; >> > + } >> >> I don't think this works. We need one last flush to not lose the last >> few pages of ram. And we need the src side sync of multifd threads to >> make sure this function doesn't return before all IO has been put on the >> wire. > > This should be the question for commit 637280aeb2, I thought it got > answered there.. It's almost what the commit message there in 637280aeb2 > wanted to justify too. Yeah, it missed the mark. > > We don't need to flush the last pages here, because we flushed it already, > in the last find_dirty_block() call where src QEMU finished scanning the > last round. Then we set complete_round=true, scan one more round, and the > iteration won't complete until the new round sees zero dirty page. Ok, let's put this in the commit message. As it stands it looks like this patch is fixing a bug with 637280aeb2 when instead it's doing further optimization, but so it happens that we still require the backward compatibility part. > > So when reaching this line, multifd send buffer must be empty. We need to > flush for each round of RAM scan, we can't avoid the flush there, but we > can save this one in complete(). > > To explain that with code, I hacked a QEMU and assert that. It ran all > fine here (this is definitely not for merge.. but to show what I meant): > > ===8<=== > diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c > index f64c4c9abd..0bd42c7627 100644 > --- a/migration/multifd-nocomp.c > +++ b/migration/multifd-nocomp.c > @@ -21,7 +21,7 @@ > #include "qemu/error-report.h" > #include "trace.h" > > -static MultiFDSendData *multifd_ram_send; > +MultiFDSendData *multifd_ram_send; > > size_t multifd_ram_payload_size(void) > { > diff --git a/migration/ram.c b/migration/ram.c > index 7284c34bd8..edeb9e28ff 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -3228,6 +3228,8 @@ out: > return done; > } > > +extern MultiFDSendData *multifd_ram_send; > + > /** > * ram_save_complete: function called to send the remaining amount of ram > * > @@ -3283,6 +3285,10 @@ static int ram_save_complete(QEMUFile *f, void *opaque) > } > } > > + if (migrate_multifd()) { > + assert(multifd_payload_empty(multifd_ram_send)); > + } > + > if (migrate_multifd() && > migrate_multifd_flush_after_each_section()) { > /* > ===8<=== > >> >> This also doesn't address what the commit message says about the recv >> side never getting the RAM_SAVE_FLAG_MULTIFD_FLUSH message. > > The new binaries now always not send RAM_SAVE_FLAG_MULTIFD_FLUSH when > complete(), however it always sends the multifd SYNC messages on the wire. > It means those SYNC messages will always arrive on dest multifd channels, > and then all these channels will wait for the main thread to collect that. > However since RAM_SAVE_FLAG_MULTIFD_FLUSH is not there, they'll hang until > multifd recv cleanups.