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Thu, 22 Feb 2024 16:08:24 -0800 (PST) Received: from x1n ([43.228.180.230]) by smtp.gmail.com with ESMTPSA id qn11-20020a17090b3d4b00b00299d4856e6asm80474pjb.1.2024.02.22.16.08.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Feb 2024 16:08:24 -0800 (PST) Date: Fri, 23 Feb 2024 08:08:14 +0800 From: Peter Xu To: Fabiano Rosas Cc: qemu-devel@nongnu.org Subject: Re: [PATCH] migration: Fix qmp_query_migrate mbps value Message-ID: References: <20240219194457.26923-1-farosas@suse.de> <87y1beascb.fsf@suse.de> <87ttm0a9t3.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87ttm0a9t3.fsf@suse.de> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -5 X-Spam_score: -0.6 X-Spam_bar: / X-Spam_report: (-0.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.002, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SORBS_WEB=1.5, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no 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 On Thu, Feb 22, 2024 at 10:49:12AM -0300, Fabiano Rosas wrote: > Peter Xu writes: > > > On Thu, Feb 22, 2024 at 05:40:41PM +0800, Peter Xu wrote: > >> On Wed, Feb 21, 2024 at 09:56:36AM -0300, Fabiano Rosas wrote: > >> > Peter Xu writes: > >> > > >> > > On Mon, Feb 19, 2024 at 04:44:57PM -0300, Fabiano Rosas wrote: > >> > >> The QMP command query_migrate might see incorrect throughput numbers > >> > >> if it runs after we've set the migration completion status but before > >> > >> migration_calculate_complete() has updated s->total_time and s->mbps. > >> > >> > >> > >> The migration status would show COMPLETED, but the throughput value > >> > >> would be the one from the last iteration and not the one from the > >> > >> whole migration. This will usually be a larger value due to the time > >> > >> period being smaller (one iteration). > >> > >> > >> > >> Move migration_calculate_complete() earlier so that the status > >> > >> MIGRATION_STATUS_COMPLETED is only emitted after the final counters > >> > >> update. > >> > >> > >> > >> Signed-off-by: Fabiano Rosas > >> > >> --- > >> > >> CI run: https://gitlab.com/farosas/qemu/-/pipelines/1182405776 > >> > >> --- > >> > >> migration/migration.c | 10 ++++++---- > >> > >> 1 file changed, 6 insertions(+), 4 deletions(-) > >> > >> > >> > >> diff --git a/migration/migration.c b/migration/migration.c > >> > >> index ab21de2cad..7486d59da0 100644 > >> > >> --- a/migration/migration.c > >> > >> +++ b/migration/migration.c > >> > >> @@ -102,6 +102,7 @@ static int migration_maybe_pause(MigrationState *s, > >> > >> int new_state); > >> > >> static void migrate_fd_cancel(MigrationState *s); > >> > >> static bool close_return_path_on_source(MigrationState *s); > >> > >> +static void migration_calculate_complete(MigrationState *s); > >> > >> > >> > >> static void migration_downtime_start(MigrationState *s) > >> > >> { > >> > >> @@ -2746,6 +2747,7 @@ static void migration_completion(MigrationState *s) > >> > >> migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE, > >> > >> MIGRATION_STATUS_COLO); > >> > >> } else { > >> > >> + migration_calculate_complete(s); > >> > >> migrate_set_state(&s->state, current_active_state, > >> > >> MIGRATION_STATUS_COMPLETED); > >> > >> } > >> > >> @@ -2784,6 +2786,7 @@ static void bg_migration_completion(MigrationState *s) > >> > >> goto fail; > >> > >> } > >> > >> > >> > >> + migration_calculate_complete(s); > >> > >> migrate_set_state(&s->state, current_active_state, > >> > >> MIGRATION_STATUS_COMPLETED); > >> > >> return; > >> > >> @@ -2993,12 +2996,15 @@ static void migration_calculate_complete(MigrationState *s) > >> > >> int64_t end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME); > >> > >> int64_t transfer_time; > >> > >> > >> > >> + /* QMP could read from these concurrently */ > >> > >> + bql_lock(); > >> > >> migration_downtime_end(s); > >> > >> s->total_time = end_time - s->start_time; > >> > >> transfer_time = s->total_time - s->setup_time; > >> > >> if (transfer_time) { > >> > >> s->mbps = ((double) bytes * 8.0) / transfer_time / 1000; > >> > >> } > >> > >> + bql_unlock(); > >> > > > >> > > The lock is not needed? > >> > > > >> > > AFAIU that was needed because of things like runstate_set() rather than > >> > > setting of these fields. > >> > > > >> > > >> > Don't we need to keep the total_time and mbps update atomic? Otherwise > >> > query-migrate might see (say) total_time=0 and mbps= or > >> > total_time= and mbps=. > >> > >> I thought it wasn't a major concern, but what you said makes sense; taking > >> it one more time doesn't really hurt after all to provide such benefit. > >> > >> > > >> > Also, what orders s->mbps update before the s->state update? I'd say we > >> > should probably hold the lock around the whole total_time,mbps,state > >> > update. > >> > >> IMHO that's fine; mutex unlock implies a RELEASE. See atomic.rst: > >> > >> - ``pthread_mutex_lock`` has acquire semantics, ``pthread_mutex_unlock`` has > >> release semantics and synchronizes with a ``pthread_mutex_lock`` for the > >> same mutex. > > > > Hmm perhaps I wrote too soon.. it should only guarantee the ordering of the > > update on the lock variable itself v.s. any previous R&Ws, nothing else. > > Only if the other side uses bql_lock() will it guarantee proper ordering. > > > > Put them in bql should work, but I hesitate such use to start using bql > > to protect state updates. > > Well, on the other hand that's a major use-case of the BQL: protecting > state that's used by QMP. > > > > > How about we drop the lock, but use an explicit smp_mb_release()? We may > > also want to use smb_load_acquire() in fill_source_migration_info() to use > > on reading &s->state (all will need some comment). To me, making sure the > > total mbps is valid seems more important; while the other races are less > > harmful, and may not be a major concern? > > That more closely reflects the problem we're trying to solve, which is > just an ordering one. However, the QMP code already holds the BQL, we > could just take benefit of that instead of adding more complex > synchronization primitives. > > May I suggest we keep it simple and move that last migrate_set_state > into the BQL as well? It's okay to me, but then let's also extend the comment a little bit on the two exact requirements we're persuing (atomicity of updating fields, ordering of state update v.s. mbps)? We can also rename migration_calculate_complete() to something like migration_completion_finalize()? Then move the state update into it. -- Peter Xu