From nobody Mon Sep 29 22:23:05 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 01942C25B0D for ; Tue, 16 Aug 2022 04:49:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232349AbiHPEtA (ORCPT ); Tue, 16 Aug 2022 00:49:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55086 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232600AbiHPEpn (ORCPT ); Tue, 16 Aug 2022 00:45:43 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D374AB5E43; Mon, 15 Aug 2022 13:44:27 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 7B555B80EAD; Mon, 15 Aug 2022 20:44:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3D82C433C1; Mon, 15 Aug 2022 20:44:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1660596265; bh=cAuQmdP8eqYQ3pb+Q6BsdvaIZxGrfLolEG77N1flqqo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ViGuAeJMpTOkE+/44PnVw/QvFIzCTZniEg942T4/9uMIy0xKiFPj6gn0s7UH7WITo QWXdftuAqDUyoNBCZY15tMHlQs8wUsOS2IczhK3BVyHuevmH1IQlRk/OUHNvztqw4l jPUrIpbOO2/doOJM+afaac+fS5f2SU55NDN5Y3VQ= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Valentin Schneider , Tianchen Ding , "Peter Zijlstra (Intel)" , Sasha Levin Subject: [PATCH 5.19 1025/1157] sched: Fix the check of nr_running at queue wakelist Date: Mon, 15 Aug 2022 20:06:21 +0200 Message-Id: <20220815180520.932083503@linuxfoundation.org> X-Mailer: git-send-email 2.37.2 In-Reply-To: <20220815180439.416659447@linuxfoundation.org> References: <20220815180439.416659447@linuxfoundation.org> User-Agent: quilt/0.67 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" From: Tianchen Ding [ Upstream commit 28156108fecb1f808b21d216e8ea8f0d205a530c ] The commit 2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling") checked rq->nr_running <=3D 1 to avoid task stacking when WF_ON_CPU. Per the ordering of writes to p->on_rq and p->on_cpu, observing p->on_cpu (WF_ON_CPU) in ttwu_queue_cond() implies !p->on_rq, IOW p has gone through the deactivate_task() in __schedule(), thus p has been accounted out of rq->nr_running. As such, the task being the only runnable task on the rq implies reading rq->nr_running =3D=3D 0 at that point. The benchmark result is in [1]. [1] https://lore.kernel.org/all/e34de686-4e85-bde1-9f3c-9bbc86b38627@linux.= alibaba.com/ Suggested-by: Valentin Schneider Signed-off-by: Tianchen Ding Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Valentin Schneider Link: https://lore.kernel.org/r/20220608233412.327341-2-dtcccc@linux.alibab= a.com Signed-off-by: Sasha Levin --- kernel/sched/core.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index b4620354767b..6f1e3eef9346 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3829,8 +3829,12 @@ static inline bool ttwu_queue_cond(int cpu, int wake= _flags) * CPU then use the wakelist to offload the task activation to * the soon-to-be-idle CPU as the current CPU is likely busy. * nr_running is checked to avoid unnecessary task stacking. + * + * Note that we can only get here with (wakee) p->on_rq=3D0, + * p->on_cpu can be whatever, we've done the dequeue, so + * the wakee has been accounted out of ->nr_running. */ - if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <=3D 1) + if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running) return true; =20 return false; --=20 2.35.1