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bh=l5xyyENtc79OotoxV9LHjiMneBMu5Xck1CB+EZ0bjwU=; b=zu9pH4Ejkh/pZtjC4eEu73ZVtf JNIQj2L5WL3GWMP2m07ki/P9wNscHIDzZiVi/Hshu6tSDjGZVUOsiaRaIsjh9KY8AP23XJjYm4Anp riVDbqpAp5Pyfy+qDXLUwWZnmzmYgkkpTbuWbJs/2XgkhmG2mIs78U5fYhoZPBTUyjFSGEWjaF6P4 Ij/gJVxJRs/ayqSLoeQNgSHV2OwNzEqPyxjVR1H1xvbIF1PHsSjauyWIa06Nlm7+c2stbsPX5z8zz WcD54Z4DaUycECX+nFupQ4/ouqt8y9RnTxiJY2JE0O8fBck0fcfI2EnLVcvD6gxSYpyAYAfvhaaOT M36nrUWA==; Received: from mcgrof by bombadil.infradead.org with local (Exim 4.96 #2 (Red Hat Linux)) id 1r43MP-0074kn-28; Fri, 17 Nov 2023 18:14:33 +0000 Date: Fri, 17 Nov 2023 10:14:33 -0800 From: Luis Chamberlain To: Ankur Arora Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at, mjguzik@gmail.com, Tejun Heo , Zefan Li , Johannes Weiner , Peter Oberparleiter , Eric Biederman , Will Deacon , Oleg Nesterov Subject: Re: [RFC PATCH 66/86] treewide: kernel: remove cond_resched() Message-ID: References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107230822.371443-1-ankur.a.arora@oracle.com> <20231107230822.371443-10-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231107230822.371443-10-ankur.a.arora@oracle.com> X-Stat-Signature: a9m97tajckq73mk7pyou1ur7esn73bze X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 88C86100016 X-Rspam-User: X-HE-Tag: 1700244887-311862 X-HE-Meta: U2FsdGVkX182VfaprnQIuW6ESdFpOYmDavhkVCPcWGOSf9evatMbTe5ej13dEVnFqnBid1/5bBtciyafEnC0fqgj4aMA3SW8BLheyrPfet0GtWVcud2Zmxb1hYS27AunzZXsitqfpElL2YBCoRbkcR3ut/392GNXFbEgF/vIynRptnfLcmuOnB9Mwke6mhmc+7t3+BZG20tSR3FoBV2REEdV9eo6yGaPBiefSWaB1iTIVmztO49QTWnIdOj0Zl6dvmAXun4tsv4XEnFXYGseuvSpdJh6OAE5xappa4fcSDTWIhy7Fj9x3f4o79MY9tCCrlh/ITzgRqIfjE88MvIAOP+rjS4ZSMnvYvLtUTHHStVoEl/6ekI5oFWY/ujwaNPK/8uxzl6sqKPpMLNVVzBtUPamKLGkjJXZZIHlZcN8spepkitW3Dw5fyASxhaMD3xX3wQBeVOLQWi0fxwn28a7S4vsPDBbqNa5vGVHCMp894iGA0iEN/AfJvTBc49zc48FN+GWckCuwXGPfwcnPKqDj6MWDbF8WrJyDlyMLIi9r6gGkZOF7iHDKdhbB4G1Zk2Gy8Hmx39eNd8JHSoUt8GpKDL+DgX5c792VLd9lUKp8JMephU/kpebl5YHT4VJfKFMQZ+nFCizoQBjO9wFlXi6fEnKBsa2Y2LoOHDtLBMTOAQzPP+YJoIBmv7hR4fZKP7fZtYop+A3DaJMLBXsZWH7v5/6aSsmzcWoyVgUu7u/tK8TQ2mBc0gYZysDrc2kg8fq+6UkbDO8EGzaAhUqYlpnbuIRypielyxJG5Rx+HQVJ2j0ElIe2FuZHVYOo18ZAUEYs+B0VmppXdQafOXGHVdVyReBx4NSsyW/lSp59JAgeq3x94bzP7J0qrTkXXDnMSoPtLxeFRUzy619yCQQklzL/YyTV/UPgDAGEb+p1gMQsxJHqzV1QEcYJyDll09tz4j/BblalMyaBnIT953+j69 XuJO6Feu 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Nov 07, 2023 at 03:08:02PM -0800, Ankur Arora wrote: > There are broadly three sets of uses of cond_resched(): > > 1. Calls to cond_resched() out of the goodness of our heart, > otherwise known as avoiding lockup splats. > > 2. Open coded variants of cond_resched_lock() which call > cond_resched(). > > 3. Retry or error handling loops, where cond_resched() is used as a > quick alternative to spinning in a tight-loop. > > When running under a full preemption model, the cond_resched() reduces > to a NOP (not even a barrier) so removing it obviously cannot matter. > > But considering only voluntary preemption models (for say code that > has been mostly tested under those), for set-1 and set-2 the > scheduler can now preempt kernel tasks running beyond their time > quanta anywhere they are preemptible() [1]. Which removes any need > for these explicitly placed scheduling points. > > The cond_resched() calls in set-3 are a little more difficult. > To start with, given it's NOP character under full preemption, it > never actually saved us from a tight loop. > With voluntary preemption, it's not a NOP, but it might as well be -- > for most workloads the scheduler does not have an interminable supply > of runnable tasks on the runqueue. > > So, cond_resched() is useful to not get softlockup splats, but not > terribly good for error handling. Ideally, these should be replaced > with some kind of timed or event wait. > For now we use cond_resched_stall(), which tries to schedule if > possible, and executes a cpu_relax() if not. > > All of these are from set-1 except for the retry loops in > task_function_call() or the mutex testing logic. > > Replace these with cond_resched_stall(). The others can be removed. > > [1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@oracle.com/ > > Cc: Tejun Heo > Cc: Zefan Li > Cc: Johannes Weiner > Cc: Peter Oberparleiter > Cc: Eric Biederman > Cc: Will Deacon > Cc: Luis Chamberlain > Cc: Oleg Nesterov > Cc: Juri Lelli > Cc: Vincent Guittot > Signed-off-by: Ankur Arora Sounds like the sort of test which should be put into linux-next to get test coverage right away. To see what really blows up. Luis