From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 2018D15E5BB for ; Tue, 17 Mar 2026 17:29:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773768578; cv=none; b=Yri6mCXuM0UroOAh6YUB8odDcjf49JZihXRwNiZ83tnNJCb4Enfm8g8y9k601duW9Oh2l7qn9SIRgHfwDhcvTIaTItrdtm4PubOBjA3GEIpcLDEZtDTeYTqHbPSrwjYe7jAdEaeua+pZvzi0d3S9hxxWrSXpQwz1ZHeGcmFLpsg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773768578; c=relaxed/simple; bh=yZzWDzduNgM9ybRvkl+o5axbf+GlUYX0BmWMHFqNg1A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=XezZUMit7lSfX4QQI58bS0ty7hAc5TDeKKILWtaQHa8b0dbt3iBr/JZTNxKRJi9uuano5+Cq+cbVXnhW3UZUoeYsdb/n/Qocsvxl7paasrTgyv01v30vu9H9d4GgFIChQxSCQoc8Fb+KU6wI8UKTIOkVxv2s1TyHnlxUyAKua/8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UEDaG8j5; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="UEDaG8j5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 79635C4CEF7; Tue, 17 Mar 2026 17:29:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773768577; bh=yZzWDzduNgM9ybRvkl+o5axbf+GlUYX0BmWMHFqNg1A=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=UEDaG8j5OnNE/95D1J01WnGf6Osb2m8pacHsAlk589H4NJzLIHrFnxrKJdAah3HkY bcUOdxkDH3moaj1HYeNV72H7i3f9Ka5mUzD56EnxdVkE8APqCPri4y45lXoz8hs/Rw LJUukIDhODanAN0Tl/EEipzOxeVFY3WL1C4uGH4pKLBS9ZzXawNMSOuJ94LJRQET0w BqcfMBmid2dQs8GoeqmkTDDNJhVyiUdo9DasVxW4bL1pYbBGiTI4z8SbWeucfou6SJ gJisfpC6MVPIIhcJVOgUm6+sfhdgt/TG0hu9wuxn+9GI7xlvNtOlTidjPqMaqkh2m8 V8kXAb8lcyYcg== From: Sasha Levin To: stable@vger.kernel.org Cc: Tejun Heo , Sasha Levin Subject: [PATCH 6.18.y] sched_ext: Fix starvation of scx_enable() under fair-class saturation Date: Tue, 17 Mar 2026 13:29:35 -0400 Message-ID: <20260317172935.242870-1-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <2026031758-devotedly-wanting-a92a@gregkh> References: <2026031758-devotedly-wanting-a92a@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tejun Heo [ Upstream commit b06ccbabe2506fd70b9167a644978b049150224a ] During scx_enable(), the READY -> ENABLED task switching loop changes the calling thread's sched_class from fair to ext. Since fair has higher priority than ext, saturating fair-class workloads can indefinitely starve the enable thread, hanging the system. This was introduced when the enable path switched from preempt_disable() to scx_bypass() which doesn't protect against fair-class starvation. Note that the original preempt_disable() protection wasn't complete either - in partial switch modes, the calling thread could still be starved after preempt_enable() as it may have been switched to ext class. Fix it by offloading the enable body to a dedicated system-wide RT (SCHED_FIFO) kthread which cannot be starved by either fair or ext class tasks. scx_enable() lazily creates the kthread on first use and passes the ops pointer through a struct scx_enable_cmd containing the kthread_work, then synchronously waits for completion. The workfn runs on a different kthread from sch->helper (which runs disable_work), so it can safely flush disable_work on the error path without deadlock. Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode") Cc: stable@vger.kernel.org # v6.12+ Signed-off-by: Tejun Heo Signed-off-by: Sasha Levin --- kernel/sched/ext.c | 66 +++++++++++++++++++++++++++++++++++++++------- 1 file changed, 56 insertions(+), 10 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 2ff7034841c7c..0f86d455f8ef7 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4600,20 +4600,30 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops) return 0; } -static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) +/* + * scx_enable() is offloaded to a dedicated system-wide RT kthread to avoid + * starvation. During the READY -> ENABLED task switching loop, the calling + * thread's sched_class gets switched from fair to ext. As fair has higher + * priority than ext, the calling thread can be indefinitely starved under + * fair-class saturation, leading to a system hang. + */ +struct scx_enable_cmd { + struct kthread_work work; + struct sched_ext_ops *ops; + int ret; +}; + +static void scx_enable_workfn(struct kthread_work *work) { + struct scx_enable_cmd *cmd = + container_of(work, struct scx_enable_cmd, work); + struct sched_ext_ops *ops = cmd->ops; struct scx_sched *sch; struct scx_task_iter sti; struct task_struct *p; unsigned long timeout; int i, cpu, ret; - if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN), - cpu_possible_mask)) { - pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n"); - return -EINVAL; - } - mutex_lock(&scx_enable_mutex); if (scx_enable_state() != SCX_DISABLED) { @@ -4830,13 +4840,15 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) atomic_long_inc(&scx_enable_seq); - return 0; + cmd->ret = 0; + return; err_free_pseqs: free_kick_pseqs(); err_unlock: mutex_unlock(&scx_enable_mutex); - return ret; + cmd->ret = ret; + return; err_disable_unlock_all: scx_cgroup_unlock(); @@ -4855,7 +4867,41 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) */ scx_error(sch, "scx_enable() failed (%d)", ret); kthread_flush_work(&sch->disable_work); - return 0; + cmd->ret = 0; +} + +static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) +{ + static struct kthread_worker *helper; + static DEFINE_MUTEX(helper_mutex); + struct scx_enable_cmd cmd; + + if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN), + cpu_possible_mask)) { + pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n"); + return -EINVAL; + } + + if (!READ_ONCE(helper)) { + mutex_lock(&helper_mutex); + if (!helper) { + helper = kthread_run_worker(0, "scx_enable_helper"); + if (IS_ERR_OR_NULL(helper)) { + helper = NULL; + mutex_unlock(&helper_mutex); + return -ENOMEM; + } + sched_set_fifo(helper->task); + } + mutex_unlock(&helper_mutex); + } + + kthread_init_work(&cmd.work, scx_enable_workfn); + cmd.ops = ops; + + kthread_queue_work(READ_ONCE(helper), &cmd.work); + kthread_flush_work(&cmd.work); + return cmd.ret; } -- 2.51.0