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sched_ext: Simplify breather mechanism with scx_aborting flag

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The breather mechanism was introduced in 62dcbab8b0 ("sched_ext: Avoid
live-locking bypass mode switching") and e32c260195 ("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.

Reviewed-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo
2025-11-11 09:18:07 -10:00
parent 61debc251c
commit a69040ed57
1 changed files with 25 additions and 29 deletions
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54
kernel/sched/ext.c
View File

@@ -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);
@@ -1831,7 +1830,7 @@ 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);
@@ -1840,9 +1839,9 @@ 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);
@@ -3741,30 +3740,14 @@ 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
@@ -3827,8 +3810,6 @@ 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()
@@ -3884,10 +3865,8 @@ 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)
@@ -3982,6 +3961,7 @@ 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:
@@ -4104,9 +4084,24 @@ done:
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))
@@ -4115,7 +4110,7 @@ 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();
@@ -4436,9 +4431,8 @@ 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;
@@ -4654,6 +4648,8 @@ 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);