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Wysocki" , Daniel Lezcano , Thomas Gleixner , Anna-Maria Behnsen Subject: [PATCH 10/10] sched/timers: Explain why idle task schedules out on remote timer enqueue Date: Fri, 11 Aug 2023 19:00:49 +0200 Message-Id: <20230811170049.308866-11-frederic@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230811170049.308866-1-frederic@kernel.org> References: <20230811170049.308866-1-frederic@kernel.org> MIME-Version: 1.0 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, RCVD_IN_DNSWL_BLOCKED,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1773955520665221724 X-GMAIL-MSGID: 1773955520665221724 Trying to avoid that didn't bring much value after testing, add comment about this. Signed-off-by: Frederic Weisbecker Acked-by: Rafael J. Wysocki --- kernel/sched/core.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c52c2eba7c73..e53b892167ad 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1135,6 +1135,28 @@ static void wake_up_idle_cpu(int cpu) if (cpu == smp_processor_id()) return; + /* + * Set TIF_NEED_RESCHED and send an IPI if in the non-polling + * part of the idle loop. This forces an exit from the idle loop + * and a round trip to schedule(). Now this could be optimized + * because a simple new idle loop iteration is enough to + * re-evaluate the next tick. Provided some re-ordering of tick + * nohz functions that would need to follow TIF_NR_POLLING + * clearing: + * + * - On most archs, a simple fetch_or on ti::flags with a + * "0" value would be enough to know if an IPI needs to be sent. + * + * - x86 needs to perform a last need_resched() check between + * monitor and mwait which doesn't take timers into account. + * There a dedicated TIF_TIMER flag would be required to + * fetch_or here and be checked along with TIF_NEED_RESCHED + * before mwait(). + * + * However, remote timer enqueue is not such a frequent event + * and testing of the above solutions didn't appear to report + * much benefits. + */ if (set_nr_and_not_polling(rq->idle)) smp_send_reschedule(cpu); else