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Message-ID: <aRGX_3UR_dPLTTrn@gpd4>
Date: Mon, 10 Nov 2025 08:45:03 +0100
From: Andrea Righi <andrea.righi@...ux.dev>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
Dan Schatzberg <schatzberg.dan@...il.com>,
Emil Tsalapatis <etsal@...a.com>, sched-ext@...ts.linux.dev,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/13] sched_ext: Simplify breather mechanism with
scx_aborting flag
On Sun, Nov 09, 2025 at 08:31:04AM -1000, Tejun Heo wrote:
> 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.
Acked-by: Andrea Righi <arighi@...dia.com>
Thanks,
-Andrea
>
> Cc: Dan Schatzberg <schatzberg.dan@...il.com>
> Cc: Emil Tsalapatis <etsal@...a.com>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
> 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
>
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