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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1r0eQp-00DMsX-3B; Wed, 08 Nov 2023 09:01:04 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id A015A30049D; Wed, 8 Nov 2023 10:01:03 +0100 (CET) Date: Wed, 8 Nov 2023 10:01:03 +0100 From: Peter Zijlstra To: Ankur Arora Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, 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 Subject: Re: [RFC PATCH 36/86] entry: irqentry_exit only preempts TIF_NEED_RESCHED Message-ID: <20231108090103.GG8262@noisy.programming.kicks-ass.net> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107215742.363031-37-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231107215742.363031-37-ankur.a.arora@oracle.com> X-Rspamd-Queue-Id: 859994000B X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: f1z663hgo7iw6ay7fei39iricmccmz8m X-HE-Tag: 1699434079-352704 X-HE-Meta: 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 uJA7d7qO Az+X5/x0KlFEIgZ/sNBcL3XXGqmE+Z7t0s8flDT4V4QvSegf8Ro28M8qIbFNoyrnQkRo9e4GEVKnClb5lxsmvJB7LmnO7ewR1gd7dmFKe2At65WwkR6NQCcsD2sJKOUvYzHCg+Eqg6OZFoxz8sGaG7X5R5/XVVIZUYYiArZZ3cdtTij6pcH/Fj7OnRhFtEumJat2viLNuMDLrKGYJAMzF4bSTsg== 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 01:57:22PM -0800, Ankur Arora wrote: > The scheduling policy for RESCHED_lazy (TIF_NEED_RESCHED_LAZY) is > to let anything running in the kernel run to completion. > Accordingly, while deciding whether to call preempt_schedule_irq() > narrow the check to tif_need_resched(RESCHED_eager). > > Also add a comment about why we need to check at all, given that we > have aleady checked the preempt_count(). > > Signed-off-by: Ankur Arora > --- > kernel/entry/common.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/kernel/entry/common.c b/kernel/entry/common.c > index 0d055c39690b..6433e6c77185 100644 > --- a/kernel/entry/common.c > +++ b/kernel/entry/common.c > @@ -384,7 +384,15 @@ void irqentry_exit_cond_resched(void) > rcu_irq_exit_check_preempt(); > if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) > WARN_ON_ONCE(!on_thread_stack()); > - if (need_resched()) > + > + /* > + * If the scheduler really wants us to preempt while returning > + * to kernel, it would set TIF_NEED_RESCHED. > + * On some archs the flag gets folded in preempt_count, and > + * thus would be covered in the conditional above, but not all > + * archs do that, so check explicitly. > + */ > + if (tif_need_resched(RESCHED_eager)) > preempt_schedule_irq(); See, I'm reading this like if we're eager to preempt, but then it's not actually eager at all and only wants to preempt when forced. This naming sucks...