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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qkMMH-00CRQV-DX; Sun, 24 Sep 2023 10:29:01 +0000 Date: Sun, 24 Sep 2023 11:29:01 +0100 From: Matthew Wilcox To: Thomas Gleixner Cc: Linus Torvalds , Peter Zijlstra , Ankur Arora , linux-kernel@vger.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, mgorman@suse.de, rostedt@goodmis.org, 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 Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED Message-ID: References: <20230911150410.GC9098@noisy.programming.kicks-ass.net> <87h6o01w1a.fsf@oracle.com> <20230912082606.GB35261@noisy.programming.kicks-ass.net> <87cyyfxd4k.ffs@tglx> <87led2wdj0.ffs@tglx> <87h6nkh5bw.ffs@tglx> <878r8wgg3b.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <878r8wgg3b.ffs@tglx> X-Rspamd-Queue-Id: EDD7018000E X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 9a4micjn6ideshjpo17q7cqq6gafmi4y X-HE-Tag: 1695551352-679958 X-HE-Meta: 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 7epiui8d Xfz5oFtvLwPtukpYi7V0Oy/uoourIFxEHw4oXdV3WWkpn4aIc1aJMSadh/5Xbg2HfQ04H94KJh+w0KAsiZCjW1cOCXyJ/j5KeE4WVqABf2uEch95vxUnGmY2vYDflb7wikr77uHX/m3XJj4kQ47V+D/E7in+u0J1o9oAslMAPjWiF/VZgA3ABg4asDw== 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: On Sun, Sep 24, 2023 at 09:55:52AM +0200, Thomas Gleixner wrote: > On Sun, Sep 24 2023 at 08:19, Matthew Wilcox wrote: > > On Sun, Sep 24, 2023 at 12:50:43AM +0200, Thomas Gleixner wrote: > >> cond_resched() cannot nest and is obviously scope-less. > >> > >> The TIF_ALLOW_RESCHED mechanism, which sparked this discussion only > >> pretends to be scoped. > >> > >> As Peter pointed out it does not properly nest with other mechanisms and > >> it cannot even nest in itself because it is boolean. > > > > We can nest a single bit without turning it into a counter -- we > > do this for memalloc_nofs_save() for example. Simply return the > > current value of the bit, and pass it to _restore(). > > Right. > > That works, but the reverse logic still does not make sense: > > allow_resched(); > .... > spin_lock(); > > while > resched_now_is_suboptimal(); > ... > spin_lock(); > > works. Oh, indeed. I had in mind state = resched_now_is_suboptimal(); spin_lock(); ... spin_unlock(); resched_might_be_optimal_again(state); ... or we could bundle it up ... state = spin_lock_resched_disable(); ... spin_unlock_resched_restore(state);