From nobody Tue Dec 2 00:03:19 2025 Received: from canpmsgout11.his.huawei.com (canpmsgout11.his.huawei.com [113.46.200.226]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 20FB3246774 for ; Tue, 25 Nov 2025 08:55:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.226 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764060926; cv=none; b=CuxgmH/zKoi+O1JlB7PIi157R+k6WS3ngiVX7pVueQhRQqpZv4pMTWNCdL9ApDdCktg1tkeXhX2StjRBhllvXOx+xE8sTCveRJ/5fszvlxAV0REmnIWgt+zuZ0/OKSkFJ12JkSCF75PF35AUbyuaGASNmgi+jHgI3S/oG2zvIKA= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764060926; c=relaxed/simple; bh=LU08/IGRxS1x/PhaCAfrV5PK5jY5u2E1ZiFiLBZh33I=; h=From:To:CC:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=AsbRG6O4z5IbYSy0mKcGH0BL+4jj+ob0xEMVK57EExNzNpJ4PXCbObX1w0bnDOBST28h15rVt1Rj7+5ImfybGfQcINKubk5pfW0ckNLUr5JcewiEC0C0EC/e0jNcfiVgO5wsflGU27PaIIinfVm/D0VAhuG2c8qylycyI8VM3Ao= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=1hXKBO10; arc=none smtp.client-ip=113.46.200.226 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="1hXKBO10" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=kOKJWCerSpOd2osfT9hJN56AwEUcm/fGyCag3PMV+zk=; b=1hXKBO10CMxfwSe5WfRVzzxq63XRD06bnQ3/AkkViY0iE8VJEGiFHnfKW3UZ4Jl5D1M/1iIiO 6G3q42LH4ka22mWLzLgKHnpyWtgLAKJxrDBqo0A0/Dm+2xPSGPtbhvpgYtUTtIPL9Bvqo809V6w H4xV5y4Dh5OtPdMXorcwvdk= Received: from mail.maildlp.com (unknown [172.19.88.163]) by canpmsgout11.his.huawei.com (SkyGuard) with ESMTPS id 4dFxLM5J3lzKm4X; Tue, 25 Nov 2025 16:53:31 +0800 (CST) Received: from kwepemg100008.china.huawei.com (unknown [7.202.181.26]) by mail.maildlp.com (Postfix) with ESMTPS id AD04418001B; Tue, 25 Nov 2025 16:55:19 +0800 (CST) Received: from huawei.com (10.67.174.242) by kwepemg100008.china.huawei.com (7.202.181.26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Tue, 25 Nov 2025 16:55:18 +0800 From: Chen Jinghuang To: CC: , , , , , , , , , Subject: [PATCH v2] sched/rt: Skip currently executing CPU in rto_next_cpu() Date: Tue, 25 Nov 2025 08:36:49 +0000 Message-ID: <20251125083649.1814558-1-chenjinghuang2@huawei.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251121123811.3d34b10b@gandalf.local.home> References: <20251121123811.3d34b10b@gandalf.local.home> 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 X-ClientProxiedBy: kwepems200002.china.huawei.com (7.221.188.68) To kwepemg100008.china.huawei.com (7.202.181.26) Content-Type: text/plain; charset="utf-8" CPU0 becomes overloaded when hosting a CPU-bound RT task, a non-CPU-bound RT task, and a CFS task stuck in kernel space. When other CPUs switch from RT to non-RT tasks, RT load balancing (LB) is triggered; with HAVE_RT_PUSH_IPI enabled, they send IPIs to CPU0 to drive the execution of rto_push_irq_work_func. During push_rt_task on CPU0, if next_task->prio < rq->donor->prio, resched_curr() sets NEED_RESCHED and after the push operation completes, CPU0 calls rto_next_cpu(). Since only CPU0 is overloaded in this scenario, rto_next_cpu() should ideally return -1 (no further IPI needed). However, multiple CPUs invoking tell_cpu_to_push() during LB increments rd->rto_loop_next. Even when rd->rto_cpu is set to -1, the mismatch between rd->rto_loop and rd->rto_loop_next forces rto_next_cpu() to restart its search from -1. With CPU0 remaining overloaded (satisfying rt_nr_migratory && rt_nr_total > 1), it gets reselected, causing CPU0 to queue irq_work to itself and send self-IPIs repeatedly. As long as CPU0 stays overloaded and other CPUs run pull_rt_tasks(), it falls into an infinite self-IPI loop, wasting CPU cycles on unnecessary interrupt handling. The triggering scenario is as follows: cpu0 cpu1 cpu2 pull_rt_task tell_cpu_to_push <------------irq_work_queue_on rto_push_irq_work_func push_rt_task resched_curr(rq) pull_rt_task rto_next_cpu tell_cpu_to_push <-------------------------- atomic_inc(rto_loop_next) rd->rto_loop !=3D next rto_next_cpu irq_work_queue_on rto_push_irq_work_func Fix redundant self-IPI by filtering the initiating CPU in rto_next_cpu(). Fixes: 4bdced5c9a292 ("sched/rt: Simplify the IPI based RT balancing logic") Suggested-by: Steven Rostedt Signed-off-by: Chen Jinghuang Reviewed-by: Steven Rostedt (Google) --- v1-->v2: - Replace the original "check NEED_RESCHED on target CPU" logic with "skip the currently executing CPU" - This modification eliminates self-IPIS --- --- kernel/sched/rt.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 7936d4333731..dc0d583aa59a 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -2100,6 +2100,7 @@ static void push_rt_tasks(struct rq *rq) */ static int rto_next_cpu(struct root_domain *rd) { + int this_cpu =3D smp_processor_id(); int next; int cpu; =20 @@ -2118,14 +2119,17 @@ static int rto_next_cpu(struct root_domain *rd) */ for (;;) { =20 - /* When rto_cpu is -1 this acts like cpumask_first() */ - cpu =3D cpumask_next(rd->rto_cpu, rd->rto_mask); - - rd->rto_cpu =3D cpu; + do { + /* When rto_cpu is -1 this acts like cpumask_first() */ + cpu =3D cpumask_next(rd->rto_cpu, rd->rto_mask); + rd->rto_cpu =3D cpu; + /* Do not send IPI to self */ + } while (cpu =3D=3D this_cpu); =20 if (cpu < nr_cpu_ids) return cpu; =20 + rd->rto_cpu =3D -1; =20 /* --=20 2.34.1