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dkim=pass header.d=linutronix.de header.s=2020e header.b=s0GXFYFi; spf=pass (imf26.hostedemail.com: domain of tglx@linutronix.de designates 193.142.43.55 as permitted sender) smtp.mailfrom=tglx@linutronix.de; dmarc=pass (policy=none) header.from=linutronix.de ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1701875422; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=DlA3J6KeiLPwS0f/3dubTvabmwmYjyMo2XWsrB4w408=; b=MNzjsD74qAA50h0STdzdvwArKMAG3gTQ3oef918zgiklRVnqFH7fjkyB6q0S29CcpPTwth veobWzZX3Rj0+oVBat/V+0AHoCkA+HN+tmMev7jBqd3EQIglgGcfe1Qj8oj2z5bQjiQLJ+ oUbqdZqf4E15zUNQJ0gbWQKpZ/VfDh0= From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1701875419; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=DlA3J6KeiLPwS0f/3dubTvabmwmYjyMo2XWsrB4w408=; b=nzFjv87JXZiNZNWNyj4RR/ie5cSZDXusw/aYYqYLsm/ikFiNJPWDmUnQQJnaxQ2uDJaS1A 2A5CqhGtwWTb0BOyCrG0yRmikNVa5Usbqz+EJktFDnFrJYgnCTQFh1aV5wf0wRNkHllni5 5RZ9pn9BKOyo5LpH6p5yFNGyE7h7docpauB4MBc9mRJZ6xxuoKK1qe3kRtVcvOLXdVA25+ 4l3m7wUW1RknCQH/GL5R9p+sLpdMaFI2WmL2ycBHpDenpdumnC5v3U2nO0qhhgjtpueFsm 0dbwJZ2Gmcn7/pOLp9N4KRjIQKrSIS3qzDQ97GHPDdKIjde0o4uX8RZYI1B5Kw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1701875419; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=DlA3J6KeiLPwS0f/3dubTvabmwmYjyMo2XWsrB4w408=; b=s0GXFYFibOv7iKdtRRl6+1zP6sqdWCve1nXTradVU1o7nofS9PlGTCqL14MjhLUGqzJnnJ bR2fhq4jnrYTlNCg== To: paulmck@kernel.org Cc: Ankur Arora , linux-kernel@vger.kernel.org, peterz@infradead.org, torvalds@linux-foundation.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 Subject: Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n In-Reply-To: <209f0e89-7ebd-4759-9883-21d842d0d26c@paulmck-laptop> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107215742.363031-49-ankur.a.arora@oracle.com> <2027da00-273d-41cf-b9e7-460776181083@paulmck-laptop> <87v89lzu5a.ffs@tglx> <209f0e89-7ebd-4759-9883-21d842d0d26c@paulmck-laptop> Date: Wed, 06 Dec 2023 16:10:18 +0100 Message-ID: <875y1bwen9.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: E6E0A140018 X-Stat-Signature: hwsc3boq8i38b3asq6sf3fgbyrzwscoh X-Rspam-User: X-HE-Tag: 1701875421-961394 X-HE-Meta: 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 WchyMg2S 7o9BnlMoN1yQiKbnFsOLQNOsjrraHr88FdASppjMmCq32fckn0eRXTIBBbtYXzioBgxXCaOZte4To41105onX5l8ljRioQ+tLQU5cAL5sgXR9gU8= 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: Paul! On Mon, Dec 04 2023 at 17:33, Paul E. McKenney wrote: > On Tue, Nov 28, 2023 at 06:04:33PM +0100, Thomas Gleixner wrote: >> So: >> >> loop() >> >> preempt_disable(); >> >> --> tick interrupt >> rcu_flavor_sched_clock_irq() >> sets NEED_RESCHED >> >> preempt_enable() >> preempt_schedule() >> schedule() >> report_QS() >> >> See? No magic nonsense in preempt_enable(), no cond_resched(), nothing. > > Understood, but that does delay detection of that quiescent state by up > to one tick. Sure, but does that really matter in practice? >> So if that turns out to matter in reality and not just by academic >> inspection, then we are far better off to annotate such code with: >> >> do { >> preempt_lazy_disable(); >> mutex_lock(); >> do_stuff(); >> mutex_unlock(); >> preempt_lazy_enable(); >> } >> >> and let preempt_lazy_enable() evaluate the NEED_RESCHED_LAZY bit. > > I am not exactly sure what semantics you are proposing with this pairing > as opposed to "this would be a good time to preempt in response to the > pending lazy request". But I do agree that something like this could > replace at least a few more instance of cond_resched(), so that is good. > Not necessarily all of them, though. The main semantic difference is that such a mechanism is properly nesting and can be eventually subsumed into the actual locking constructs. >> Just insisting that RCU_PREEMPT=n requires cond_resched() and whatsoever >> is not really getting us anywhere. > > Except that this is not what is happening, Thomas. ;-) > > You are asserting that all of the cond_resched() calls can safely be > eliminated. That might well be, but more than assertion is required. > You have come up with some good ways of getting rid of some classes of > them, which is a very good and very welcome thing. But that is not the > same as having proved that all of them may be safely removed. Neither have you proven that any of them will be required with the new PREEMPT_LAZY model. :) Your experience and knowledge in this area is certainly appreciated, but under the changed semantics of LAZY it's debatable whether observations and assumptions which are based on PREEMPT_NONE behaviour still apply. We'll see. Thanks, tglx