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 763512309AA; Sun, 9 Nov 2025 18:31:19 +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=1762713079; cv=none; b=Nl84erxO8BZvd23i8P7WPJDkJyrqQIvzl3yvKPBi6KA5FYqSn8iDCDQcpxOaN7/hF5Q+QaV8/XC7A873qVAGHaW5BwvOosNuEUw3p49gVcRhlW/oOsDTaNodfcIYrUF75bu4u9y7yUvMwOwjKTCwuFC9X60zzh34SLPyBsIDYeE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762713079; c=relaxed/simple; bh=tTtfTAtF+MEdRbabPBiUCDNvraUnRq+GL8nYVYsCQy0=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=X3lvdpl1CkNOqKhcVmEOMdCNV0qeP9xu3UEzr/lOq/FaZ1g6d6aQxHbqg/qcx5QkDddI61Zfv+ixlzDrLdbOJf2RtJ0XAGzpAB1MNteB//szGmru+3CqT/vlbkCBRCxb3ANBzoGNO5ErfOyJe5aWjOc9gEk0mGffpPCYBqZpnbE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R20d2KJe; 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="R20d2KJe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 220E6C16AAE; Sun, 9 Nov 2025 18:31:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1762713079; bh=tTtfTAtF+MEdRbabPBiUCDNvraUnRq+GL8nYVYsCQy0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=R20d2KJeS4NJShSP+SxO63OkPvzdO9MYvThOm7iaTUB51BjS0TfvjUeRXomaUC5UN M9N+xL7hNcUa/yi5/qIEaH0Tz+GkgbY8GTqVcQ36XwdA7f0UUZJC2v0ZdM6JuhD9ny vUVSciP9EkCNuHSWnzXjoEuj4WSQc5SotdopRrPtINZ44S9WbfrvtwC8KO40pnBGdm vasDxqm8IvUmO+QGADkzHzhmzp7e7Vj8XWOW05SViZ8d9bwFI9xkRM98tuoOwX1Evn XUNuQh6F711KAcyrBVLiL4vhlRFSiy3AQbcn6hIbNFf9vBQ/UICigV7EjAMymP8N9Q Phe8S3hCxZOsg== From: Tejun Heo To: David Vernet , Andrea Righi , Changwoo Min Cc: Dan Schatzberg , Emil Tsalapatis , sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org, Tejun Heo Subject: [PATCH 05/13] sched_ext: Simplify breather mechanism with scx_aborting flag Date: Sun, 9 Nov 2025 08:31:04 -1000 Message-ID: <20251109183112.2412147-6-tj@kernel.org> X-Mailer: git-send-email 2.51.1 In-Reply-To: <20251109183112.2412147-1-tj@kernel.org> References: <20251109183112.2412147-1-tj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The breather mechanism was introduced in 62dcbab8b0ef ("sched_ext: Avoid live-locking bypass mode switching") and e32c260195e6 ("sched_ext: Enable the ops breather and eject BPF scheduler on softlockup") to prevent live-locks by injecting delays when CPUs are trapped in dispatch paths. Currently, it uses scx_breather_depth (atomic_t) and scx_in_softlockup (unsigned long) with separate increment/decrement and cleanup operations. The breather is only activated when aborting, so tie it directly to the exit mechanism. Replace both variables with scx_aborting flag set when exit is claimed and cleared after bypass is enabled. Introduce scx_claim_exit() to consolidate exit_kind claiming and breather enablement. This eliminates scx_clear_softlockup() and simplifies scx_softlockup() and scx_bypass(). The breather mechanism will be replaced by a different abort mechanism in a future patch. This simplification prepares for that change. Cc: Dan Schatzberg Cc: Emil Tsalapatis Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 54 +++++++++++++++++++++------------------------- 1 file changed, 25 insertions(+), 29 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 4b8b91494947..905d01f74687 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -33,9 +33,8 @@ static DEFINE_MUTEX(scx_enable_mutex); DEFINE_STATIC_KEY_FALSE(__scx_enabled); DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem); static atomic_t scx_enable_state_var = ATOMIC_INIT(SCX_DISABLED); -static unsigned long scx_in_softlockup; -static atomic_t scx_breather_depth = ATOMIC_INIT(0); static int scx_bypass_depth; +static bool scx_aborting; static bool scx_init_task_enabled; static bool scx_switching_all; DEFINE_STATIC_KEY_FALSE(__scx_switched_all); @@ -1834,7 +1833,7 @@ static void scx_breather(struct rq *rq) lockdep_assert_rq_held(rq); - if (likely(!atomic_read(&scx_breather_depth))) + if (likely(!READ_ONCE(scx_aborting))) return; raw_spin_rq_unlock(rq); @@ -1843,9 +1842,9 @@ static void scx_breather(struct rq *rq) do { int cnt = 1024; - while (atomic_read(&scx_breather_depth) && --cnt) + while (READ_ONCE(scx_aborting) && --cnt) cpu_relax(); - } while (atomic_read(&scx_breather_depth) && + } while (READ_ONCE(scx_aborting) && time_before64(ktime_get_ns(), until)); raw_spin_rq_lock(rq); @@ -3740,30 +3739,14 @@ void scx_softlockup(u32 dur_s) goto out_unlock; } - /* allow only one instance, cleared at the end of scx_bypass() */ - if (test_and_set_bit(0, &scx_in_softlockup)) - goto out_unlock; - printk_deferred(KERN_ERR "sched_ext: Soft lockup - CPU%d stuck for %us, disabling \"%s\"\n", smp_processor_id(), dur_s, scx_root->ops.name); - /* - * Some CPUs may be trapped in the dispatch paths. Enable breather - * immediately; otherwise, we might even be able to get to scx_bypass(). - */ - atomic_inc(&scx_breather_depth); - scx_error(sch, "soft lockup - CPU#%d stuck for %us", smp_processor_id(), dur_s); out_unlock: rcu_read_unlock(); } -static void scx_clear_softlockup(void) -{ - if (test_and_clear_bit(0, &scx_in_softlockup)) - atomic_dec(&scx_breather_depth); -} - /** * scx_bypass - [Un]bypass scx_ops and guarantee forward progress * @bypass: true for bypass, false for unbypass @@ -3826,8 +3809,6 @@ static void scx_bypass(bool bypass) ktime_get_ns() - bypass_timestamp); } - atomic_inc(&scx_breather_depth); - /* * No task property is changing. We just need to make sure all currently * queued tasks are re-queued according to the new scx_rq_bypassing() @@ -3883,10 +3864,8 @@ static void scx_bypass(bool bypass) raw_spin_rq_unlock(rq); } - atomic_dec(&scx_breather_depth); unlock: raw_spin_unlock_irqrestore(&bypass_lock, flags); - scx_clear_softlockup(); } static void free_exit_info(struct scx_exit_info *ei) @@ -3981,6 +3960,7 @@ static void scx_disable_workfn(struct kthread_work *work) /* guarantee forward progress by bypassing scx_ops */ scx_bypass(true); + WRITE_ONCE(scx_aborting, false); switch (scx_set_enable_state(SCX_DISABLING)) { case SCX_DISABLING: @@ -4103,9 +4083,24 @@ static void scx_disable_workfn(struct kthread_work *work) scx_bypass(false); } -static void scx_disable(enum scx_exit_kind kind) +static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind) { int none = SCX_EXIT_NONE; + + if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind)) + return false; + + /* + * Some CPUs may be trapped in the dispatch paths. Enable breather + * immediately; otherwise, we might not even be able to get to + * scx_bypass(). + */ + WRITE_ONCE(scx_aborting, true); + return true; +} + +static void scx_disable(enum scx_exit_kind kind) +{ struct scx_sched *sch; if (WARN_ON_ONCE(kind == SCX_EXIT_NONE || kind == SCX_EXIT_DONE)) @@ -4114,7 +4109,7 @@ static void scx_disable(enum scx_exit_kind kind) rcu_read_lock(); sch = rcu_dereference(scx_root); if (sch) { - atomic_try_cmpxchg(&sch->exit_kind, &none, kind); + scx_claim_exit(sch, kind); kthread_queue_work(sch->helper, &sch->disable_work); } rcu_read_unlock(); @@ -4435,9 +4430,8 @@ static void scx_vexit(struct scx_sched *sch, const char *fmt, va_list args) { struct scx_exit_info *ei = sch->exit_info; - int none = SCX_EXIT_NONE; - if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind)) + if (!scx_claim_exit(sch, kind)) return; ei->exit_code = exit_code; @@ -4653,6 +4647,8 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) */ WARN_ON_ONCE(scx_set_enable_state(SCX_ENABLING) != SCX_DISABLED); WARN_ON_ONCE(scx_root); + if (WARN_ON_ONCE(READ_ONCE(scx_aborting))) + WRITE_ONCE(scx_aborting, false); atomic_long_set(&scx_nr_rejected, 0); -- 2.51.1