From nobody Tue Dec 30 16:38:20 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BFB41C4332F for ; Tue, 14 Nov 2023 19:39:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231874AbjKNTix (ORCPT ); Tue, 14 Nov 2023 14:38:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53292 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231694AbjKNTiv (ORCPT ); Tue, 14 Nov 2023 14:38:51 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 53BF210D for ; Tue, 14 Nov 2023 11:38:48 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 219ACC433C7; Tue, 14 Nov 2023 19:38:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1699990727; bh=TiiyIUwqEqNWx33Q4JpkMzZZTKBz7XGXy64YGa5f2BI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=BJCok1LbgCXsmVwz+wvwuwA+GtIUohmHHu9CjSL0aGZe6xFbQwsvWhCAEvG5pOHZm +g6RREpNN+IcN3bU0RHg1S9tWVOtmP9QQ5SVqJp0PDEmjhgrh+1haH4b1LrXE9j05r PWwDMvHiOdJ1BSJhpqg/ixBHCGfQ606CufxQ7BD95I/Kk35lkIwP03b4qb0/0KAWgy 5luM9/GwumxshVaRAwlLHDglSTj3JtEKby3SnUHdYAiy2LSZS/0gtMellSmYlCUWvk cLYSbvN6TDMB8b+uy0TFtvInwWUCkxH9Th3iV179IeDBc27MztCMriYsHTN0uC0SG/ 0hIl//2IHIw0Q== From: Frederic Weisbecker To: Peter Zijlstra Cc: LKML , Frederic Weisbecker , "Rafael J . Wysocki" , Daniel Lezcano , Thomas Gleixner , Anna-Maria Behnsen , Ingo Molnar Subject: [PATCH 2/2] sched/timers: Explain why idle task schedules out on remote timer enqueue Date: Tue, 14 Nov 2023 14:38:40 -0500 Message-ID: <20231114193840.4041-3-frederic@kernel.org> X-Mailer: git-send-email 2.42.1 In-Reply-To: <20231114193840.4041-1-frederic@kernel.org> References: <20231114193840.4041-1-frederic@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Trying to avoid that didn't bring much value after testing, add comment about this. Acked-by: Rafael J. Wysocki Signed-off-by: Frederic Weisbecker --- kernel/sched/core.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a708d225c28e..50abc7eddb82 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1131,6 +1131,28 @@ static void wake_up_idle_cpu(int cpu) if (cpu =3D=3D smp_processor_id()) return; =20 + /* + * 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 --=20 2.42.1