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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7ae3783d086sm510953885a.107.2024.10.01.08.06.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Oct 2024 08:06:50 -0700 (PDT) Date: Tue, 1 Oct 2024 11:06:47 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , Jiang Jiacheng Cc: Markus Armbruster , qemu-devel@nongnu.org, Prasad Pandit , Julia Suvorova , Fabiano Rosas , Juraj Marcin , "Dr . David Alan Gilbert" , Jiang Jiacheng Subject: Re: [PATCH 0/7] migration: query-migrationthreads enhancements and cleanups Message-ID: References: <20240930195837.825728-1-peterx@redhat.com> <87o744e5pa.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.144, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: 2.1.29 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 Tue, Oct 01, 2024 at 03:25:14PM +0100, Daniel P. Berrangé wrote: > On Tue, Oct 01, 2024 at 07:46:09AM +0200, Markus Armbruster wrote: > > Command query-migrationthreads went in without a QAPI ACK. Issues > > review should have caught: > > > > * Flawed documentation. Fixed in commit e6c60bf02d1. > > > > * It should have been spelled query-migration-threads. Not worth fixing > > now, I guess. > > > > * What are the use cases? The commit message doesn't tell! If it's > > just for debugging, the command should be marked unstable. > > It is hard to use too. > > Lets say a mgmt app wants to restrict migration threads to some > certain pCPUs. It can't call query-migrationthreads beforehand > as the threads don't exist until migration is started. If it > calls after migration is started, then there's a window where > threads are running on arbitrary pCPUs that QEMU has access > to. There's no synchronization point where threads have been > created & can be queried, but are not yet sending data (and > thus burning CPU time) Indeed, I suppose tricks needed if to work with such model, e.g., mgmt needs to turn bw=0, start migration, query TIDs, then restore bw. However that still lacks at least the dest multifd threads, as currently it only reports src multifd threads TIDs. I don't see why a serious mgmt would like to pin and care only src threads, not dest threads, which can also eat as much (or even more) pCPU resources. For real debugging purpose, I actually don't see a major value out of it either, because GDB can provide all information that this API wants to provide, and only better with thread stacks if we want. Since I don't see how this can be used right, it didn't get proper QAPI reviews, and further I highly suspect whether this API is consumed by anyone at all.. in any serious way. Shall we remove this API (with/without going through the deprecation process)? I added the author Jiacheng too. Thanks, -- Peter Xu