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Wed, 27 Oct 2021 18:56:48 -0700 (PDT) MIME-Version: 1.0 References: <20211009075612.230283-1-leobras@redhat.com> <20211009075612.230283-4-leobras@redhat.com> <20211011193156.biedorxjetduaf7y@redhat.com> <87sfx6g55m.fsf@dusky.pond.sub.org> In-Reply-To: <87sfx6g55m.fsf@dusky.pond.sub.org> From: Leonardo Bras Soares Passos Date: Wed, 27 Oct 2021 22:56:51 -0300 Message-ID: Subject: Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy) To: Markus Armbruster Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lsoaresp@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Received-SPF: pass client-ip=170.10.133.124; envelope-from=lsoaresp@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= , Juan Quintela , Jason Wang , "Dr. David Alan Gilbert" , Peter Xu , qemu-devel , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hello Markus! (comments at the bottom) On Tue, Oct 12, 2021 at 2:54 AM Markus Armbruster wrote: > > Leonardo Bras Soares Passos writes: > > > Hello Eric, > > > > On Mon, Oct 11, 2021 at 4:32 PM Eric Blake wrote: > >> > >> On Sat, Oct 09, 2021 at 04:56:13AM -0300, Leonardo Bras wrote: > >> > Implement zerocopy on nocomp_send_write(), by making use of QIOChannel > >> > zerocopy interface. > >> > > >> > Change multifd_send_sync_main() so it can distinguish the last sync from > >> > the setup and per-iteration ones, so a flush_zerocopy() can be called > >> > at the last sync in order to make sure all RAM is sent before finishing > >> > the migration. > >> > > >> > Also make it return -1 if flush_zerocopy() fails, in order to cancel > >> > the migration process, and avoid resuming the guest in the target host > >> > without receiving all current RAM. > >> > > >> > This will work fine on RAM migration because the RAM pages are not usually freed, > >> > and there is no problem on changing the pages content between async_send() and > >> > the actual sending of the buffer, because this change will dirty the page and > >> > cause it to be re-sent on a next iteration anyway. > >> > > >> > Given a lot of locked memory may be needed in order to use multid migration > >> > with zerocopy enabled, make it optional by creating a new parameter > >> > multifd-zerocopy on qapi, so low-privileged users can still perform multifd > >> > migrations. > >> > > >> > Signed-off-by: Leonardo Bras > >> > --- > >> > qapi/migration.json | 18 ++++++++++++++++++ > >> > migration/migration.h | 1 + > >> > migration/multifd.h | 2 +- > >> > migration/migration.c | 20 ++++++++++++++++++++ > >> > migration/multifd.c | 33 ++++++++++++++++++++++++++++----- > >> > migration/ram.c | 20 +++++++++++++------- > >> > monitor/hmp-cmds.c | 4 ++++ > >> > 7 files changed, 85 insertions(+), 13 deletions(-) > >> > > >> > diff --git a/qapi/migration.json b/qapi/migration.json > >> > index 88f07baedd..c4890cbb54 100644 > >> > --- a/qapi/migration.json > >> > +++ b/qapi/migration.json > >> > @@ -724,6 +724,11 @@ > >> > # will consume more CPU. > >> > # Defaults to 1. (Since 5.0) > >> > # > >> > +# @multifd-zerocopy: Controls behavior on sending memory pages on multifd migration. > >> > +# When true, enables a zerocopy mechanism for sending memory > >> > +# pages, if host does support it. > >> > >> s/does support/supports/ (several instances this patch) > > > > I will make sure to fix that in v5. > > > >> > >> > +# Defaults to false. (Since 6.2) > >> > +# > >> > # @block-bitmap-mapping: Maps block nodes and bitmaps on them to > >> > # aliases for the purpose of dirty bitmap migration. Such > >> > # aliases may for example be the corresponding names on the > >> > @@ -758,6 +763,7 @@ > >> > 'xbzrle-cache-size', 'max-postcopy-bandwidth', > >> > 'max-cpu-throttle', 'multifd-compression', > >> > 'multifd-zlib-level' ,'multifd-zstd-level', > >> > + 'multifd-zerocopy', > >> > 'block-bitmap-mapping' ] } > >> > >> Should this feature be guarded with 'if':'CONFIG_LINUX', since that's > >> the only platform where you even compile the code (even then, it can > >> still fail if the kernel doesn't support it). > > > > I think it makes sense for the feature to always be available, even > > though it's not supported > > outside linux > v4.14. > > > > IMHO it makes more sense for the user to get an error when it starts > > migration, due to host > > not supporting zerocopy, than the error happening in the config step, > > which may cause the user > > to question if it was the right parameter. > > > > The config message error here could also be ignored, and users can > > think zerocopy is working, while it's not. > > > > For automated migrations, this feature should never be enabled for > > non-linux / older linux hosts anyway. > > > > Is there a good argument I am missing for this feature being disabled > > on non-linux? > > The general argument for having QAPI schema 'if' mirror the C > implementation's #if is introspection. Let me explain why that matters. > > Consider a management application that supports a range of QEMU > versions, say 5.0 to 6.2. Say it wants to use an QMP command that is > new in QEMU 6.2. The sane way to do that is to probe for the command > with query-qmp-schema. Same for command arguments, and anything else > QMP. > > If you doubt "sane", check out Part II of "QEMU interface introspection: > From hacks to solutions"[*]. > > The same technique works when a QMP command / argument / whatever is > compile-time conditional ('if' in the schema). The code the management > application needs anyway to deal with older QEMU now also deals with > "compiled out". Nice. > > Of course, a command or argument present in QEMU can still fail, and the > management application still needs to handle failure. Distinguishing > different failure modes can be bothersome and/or fragile. > > By making the QAPI schema conditional mirror the C conditional, you > squash the failure mode "this version of QEMU supports it, but this > build of QEMU does not" into "this version of QEMU does not support > it". Makes sense, doesn't it? > > A minor additional advantage is less generated code. > > > > [*] http://events17.linuxfoundation.org/sites/events/files/slides/armbru-qemu-introspection.pdf > This was very informative, thanks! I now understand the rationale about this choice. TBH I am not very used to this syntax. I did a take a peek at some other json files, and ended adding this lines in code, which compiled just fine: for : enum MigrationParameter {'name': 'multifd-zerocopy', 'if' : 'CONFIG_LINUX'}, for : struct MigrateSetParameters and struct MigrationParameters: '*multifd-zerocopy': { 'type': 'bool', 'if': 'CONFIG_LINUX' }, Is that enough? Is there any other necessary change? Thanks for reviewing and for helping out with this! Best regards, Leonardo Bras