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[46.135.72.140]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-abf0c0b99a6sm199241366b.13.2025.02.27.15.37.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Feb 2025 15:37:47 -0800 (PST) From: Mateusz Guzik To: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, rostedt@goodmis.org Cc: vschneid@redhat.com, mgorman@suse.de, bsegall@google.com, dietmar.eggemann@arm.com, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org, Mateusz Guzik Subject: [RFC PATCH] wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event() Date: Fri, 28 Feb 2025 00:36:52 +0100 Message-ID: <20250227233652.142613-1-mjguzik@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In vast majority of cases the condition determining whether the thread can proceed is true after the first wake up. However, even in that there is an additional call to prepare_to_wait_event(= ), resulting in a spurious irq + lock trip. Then it calls into finish_wait() to unlink itself. pre-check the condition instead after waking up instead. Stats gathared during a kernel build: bpftrace -e 'kprobe:prepare_to_wait_event,kprobe:finish_wait \ { @[probe] =3D count(); }' @[kprobe:finish_wait]: 392483 @[kprobe:prepare_to_wait_event]: 778690 As in calls to prepare_to_wait_event() almost double calls to finish_wait(). This evens out with the patch. Signed-off-by: Mateusz Guzik --- One may worry about using "condition" twice. However, macros leading up to this one already do it, so it should be fine. Also one may wonder about fences -- to my understanding going off and on CPU guarantees a full fence. include/linux/wait.h | 3 +++ 1 file changed, 3 insertions(+) diff --git a/include/linux/wait.h b/include/linux/wait.h index 2bdc8f47963b..965a19809c7e 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -316,6 +316,9 @@ extern void init_wait_entry(struct wait_queue_entry *wq= _entry, int flags); } \ \ cmd; \ + \ + if (condition) \ + break; \ } \ finish_wait(&wq_head, &__wq_entry); \ __out: __ret; \ --=20 2.43.0