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 3D9AB3B27FB; Mon, 23 Mar 2026 14:03:07 +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=1774274587; cv=none; b=EQmmyacEfgeBEnc66I/K1tJ/E+oSCl8HiL4L5DSeYUnUdPQkiXUHqjvCrInGJVVXy3/pt3bXi4wUi6IRE0hnkVVW/Qu51oDRsFKid+yY+BRepd/9/MXF/9ehuJQv8FBdsPWgTkhSfjT2hpVVcmMMo74oyCkLrNG2ER4oagFS95s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774274587; c=relaxed/simple; bh=V4m2wKHCm7HLn2PQLYraIo9oPGErUOUtq3Ysgra3DCM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=BO3Wr8BQ+Drq80Z8sf+iK9q3wmSE2DI49o79GRCr+hbn/rwRXkckHDAoSiewv1ludlWb54NYL33P6yOcw8yiw+iqJOYQkBKGkiWiEzgXpd06h+uhby3aW1K7dUphv44kaSupl5tQl+272mbjVJCWlcNVr/VLWizFNWSxJxZf+vA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Y49nD/y/; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="Y49nD/y/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A9843C4CEF7; Mon, 23 Mar 2026 14:03:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774274587; bh=V4m2wKHCm7HLn2PQLYraIo9oPGErUOUtq3Ysgra3DCM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Y49nD/y/I8Joj09o8fPZBPSy6pvxRmDdc1fIRp3Q95VlGtXXPMqqHrN6A4N51zsee hkG6UFePNUzD0DeCCJw1MQYctqLB9P+yqUnQyQoPmYxrkrlQ3vHLgVH5whZd0BrIIi IdTs+YylIhzDLP9EJglPL9YMZzZ8gWgc69+gj2Hw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Tejun Heo , Sasha Levin Subject: [PATCH 6.18 045/212] sched_ext: Fix starvation of scx_enable() under fair-class saturation Date: Mon, 23 Mar 2026 14:44:26 +0100 Message-ID: <20260323134505.190598580@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134503.770111826@linuxfoundation.org> References: <20260323134503.770111826@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ 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 Signed-off-by: Greg Kroah-Hartman --- kernel/sched/ext.c | 66 ++++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 56 insertions(+), 10 deletions(-) --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4598,20 +4598,30 @@ static int validate_ops(struct scx_sched 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) { @@ -4828,13 +4838,15 @@ static int scx_enable(struct sched_ext_o 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(); @@ -4853,7 +4865,41 @@ err_disable: */ 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; }