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 B56A035F5E9 for ; Tue, 17 Mar 2026 11:36:01 +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=1773747361; cv=none; b=O+cZzgfYYEn+EvoBVbowpBKnYtgpwBwMsYQLBPI1H0qz4bzbClGpKmiAHkMV9pk5F1rj2cbQsa72ssEaRvKuEeNSPvoHhM65WTVM2CUDohq5ctOpArTMRPyqjJIkkOZKaC1WbfVRzJoj8tKX7ouMopmFK++1I17bwYbYryO+LI0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773747361; c=relaxed/simple; bh=6UPGEtykE4dEvaHLG41YqkipBwckLLabqyhU/OZUZNU=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=scLs9m4rHIq0S8tVFgushvT0GIogtdrxgHxNxZTluaCToDWIfy7agU8+YEFR0pbx9Z8J+/qMA0ftKzVbQgOLaFWLUcBk1AR3w75zGSot4a+F2JAm5zIK9Sy1GCiFp0tDxB+nzt1DL0Hpa+ve6S2uP25IkaKPqNZiek9TM2osyYA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=NI5F8C8d; 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="NI5F8C8d" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB1E7C2BC9E; Tue, 17 Mar 2026 11:36:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1773747361; bh=6UPGEtykE4dEvaHLG41YqkipBwckLLabqyhU/OZUZNU=; h=Subject:To:Cc:From:Date:From; b=NI5F8C8dSZlV2BYW0viPaasz8Yt+spgEARPdvyJHczCXSOpXlofwVVq7IirTpBq8L Lbjv6NuegKCPiM4/qoJ/vO4N8cfd0fBItBTkdeuQ7Nmc+GQO8weI/FK4J8KoVydJuW O+SXUg7ZaHEIpjFapyNIzhjJHoNH+1KM1wcMshBk= Subject: FAILED: patch "[PATCH] sched_ext: Fix starvation of scx_enable() under fair-class" failed to apply to 6.18-stable tree To: tj@kernel.org Cc: From: Date: Tue, 17 Mar 2026 12:35:58 +0100 Message-ID: <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-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.18-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y git checkout FETCH_HEAD git cherry-pick -x b06ccbabe2506fd70b9167a644978b049150224a # git commit -s git send-email --to '' --in-reply-to '2026031758-devotedly-wanting-a92a@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^.. Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From b06ccbabe2506fd70b9167a644978b049150224a Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 3 Mar 2026 01:01:15 -1000 Subject: [PATCH] sched_ext: Fix starvation of scx_enable() under fair-class saturation 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 diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index eab6e09b6442..ba51969718f5 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4975,20 +4975,30 @@ 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) { @@ -5205,13 +5215,15 @@ 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_ksyncs: free_kick_syncs(); err_unlock: mutex_unlock(&scx_enable_mutex); - return ret; + cmd->ret = ret; + return; err_disable_unlock_all: scx_cgroup_unlock(); @@ -5230,7 +5242,41 @@ 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; }