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bh=ARFYRZtJ6JOcPNLZxUHFjYKm8iYuaCTaRJaLSU6X0OU=; b=AsQoiKZW+wBQTzPvvHzqHA4iZ6B+zH0uqo/NLzBosq+d6mCkO1lSM9NGw1h9XiHGq3r42h 1YiE+zIfe5haoE3vXJ6IQfAWob9enS5W2FI+ik0h7AkQlFTkYqs8pPCL+sB7RC6st9y/9J uJvqwFhWHUg+YYMG6RPQ+3uwhrj7LH/CFlRlzek6LII1YDP8ACprFyP0L6gqdOK8f0XHla dPS8uQ5RUWKiOKSYYVefCqZpA7ZXSNc2PAOa4W5p9H/EPr/RCQSIvWxemXwLiGU682Mjrp tmaGBemGjfXZ5geSTkNQ3FDbFNmwvCLan3mVn2UK0h5QK2wAGBtgBujOYEB5Ow== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1699528775; 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=ARFYRZtJ6JOcPNLZxUHFjYKm8iYuaCTaRJaLSU6X0OU=; b=wcjPJI03vhFC3DvSrk9NFzTiVoVaq5A9h4g1/Pg5f93lbpwGOuqmolJJQ9kVL/PIFYxvV+ fqz0xFpYFUd7xNAw== To: Ankur Arora , linux-kernel@vger.kernel.org Cc: 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, Ankur Arora Subject: Re: [RFC PATCH 54/86] sched: add cond_resched_stall() In-Reply-To: <20231107215742.363031-55-ankur.a.arora@oracle.com> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107215742.363031-55-ankur.a.arora@oracle.com> Date: Thu, 09 Nov 2023 12:19:34 +0100 Message-ID: <87cywjfa2h.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain X-Rspam-User: X-Stat-Signature: 6gmgg6dxet1cxpr8iu6g8gqp7mec86k3 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: CBDE6A0025 X-HE-Tag: 1699528777-134756 X-HE-Meta: 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 DCBY/baA JHapjMDGAe2a5MywxanlzMXswLcsooh1gW8+YYDrZal6IlbZFtkExVPoYhKnT+P5Qpf1juhDPN+x/sF8d8e0t1j8NkpAF2QxzJKOD 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 13:57, Ankur Arora wrote: > The kernel has a lot of intances of cond_resched() where it is used > as an alternative to spinning in a tight-loop while waiting to > retry an operation, or while waiting for a device state to change. > > Unfortunately, because the scheduler is unlikely to have an > interminable supply of runnable tasks on the runqueue, this just > amounts to spinning in a tight-loop with a cond_resched(). > (When running in a fully preemptible kernel, cond_resched() > calls are stubbed out so it amounts to even less.) > > In sum, cond_resched() in error handling/retry contexts might > be useful in avoiding softlockup splats, but not very good at > error handling. Ideally, these should be replaced with some kind > of timed or event wait. > > For now add cond_resched_stall(), which tries to schedule if > possible, and failing that executes a cpu_relax(). What's the point of this new variant of cond_resched()? We really do not want it at all. > +int __cond_resched_stall(void) > +{ > + if (tif_need_resched(RESCHED_eager)) { > + __preempt_schedule(); Under the new model TIF_NEED_RESCHED is going to reschedule if the preemption counter goes to zero. So the typical while (readl(mmio) & BUSY) cpu_relax(); will just be preempted like any other loop, no? Confused.