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David Alan Gilbert" , Julia Suvorova , Jiang Jiacheng Subject: Re: [PATCH] migration: Remove interface query-migrationthreads In-Reply-To: References: <20241011153417.516715-1-peterx@redhat.com> Date: Mon, 14 Oct 2024 11:22:21 -0300 Message-ID: <87msj6rcgy.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; ARC_NA(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; MISSING_XM_UA(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_SEVEN(0.00)[9]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:mid] Received-SPF: pass client-ip=195.135.223.131; 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, 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 Daniel P. Berrang=C3=A9 writes: > On Fri, Oct 11, 2024 at 11:34:17AM -0400, Peter Xu wrote: >> This reverts two commits: >>=20 >> 671326201dac8fe91222ba0045709f04a8ec3af4 >> 1b1f4ab69c41279a45ccd0d3178e83471e6e4ec1 >>=20 >> Meanwhile it adds an entry to removed-features.rst for the >> query-migrationthreads QMP command. >>=20 >> This patch originates from another patchset [1] that wanted to cleanup t= he >> interface and add corresponding HMP command, as lots of things are missi= ng >> in the query report; so far it only reports the main thread and multifd >> sender threads; all the rest migration threads are not reported, includi= ng >> multifd recv threads. >>=20 >> As pointed out by Dan in the follow up discussions [1], the API is desig= ned >> in an awkward way where CPU pinning may not cover the whole lifecycle of >> even the thread being reported. When asked, we also didn't get chance to >> hear from the developer who introduced this feature to explain how this = API >> can be properly used. >>=20 >> OTOH, this feature from debugging POV isn't very helpful either, as all >> these information can be easily obtained by GDB. Esepcially, if with >> "-name $VM,debug-threads=3Don" we do already have names for each migrati= on >> threads (which covers more than multifd sender threads). >>=20 >> So it looks like the API isn't helpful in any form as of now, besides it >> only adds maintenance burden to migration code, even if not much. >>=20 >> Considering that so far there's totally no justification on how to use t= his >> interface correctly, let's remove this interface instead of cleaning it = up. >>=20 >> In this special case, we even go beyond normal deprecation procedure, >> because a deprecation process would only make sense when there are exist= ing >> users. In this specific case, we expect zero serious users with this API. > > We have no way of knowing whether there are existing users of this, or > any other feature in QEMU. This is why we have a formal deprecation > period, rather than immediately deleting existing features. > > Yes, there are plenty of reasons why this feature is sub-optimal, but > it is not broken to the extent that it is *impossible* for people to > be using it. > > IOW, I don't see that there's anything special here to justify bypassing > our deprecation process here. I have no dog in this race, but as a data point, I see that this was submitted to libvirt as a new migrationpin command: https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/FVNA= UEVIMLG6F2VCRKHZDUEOLBJCXQHO/#BVEGJVZMMLQMXE263GO5BSIWUDIYIFZU > > > With regards, > Daniel