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sched_ext: Avoid live-locking bypass mode switching

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A poorly behaving BPF scheduler can live-lock the system by e.g. incessantly
banging on the same DSQ on a large NUMA system to the point where switching
to the bypass mode can take a long time. Turning on the bypass mode requires
dequeueing and re-enqueueing currently runnable tasks, if the DSQs that they
are on are live-locked, this can take tens of seconds cascading into other
failures. This was observed on 2 x Intel Sapphire Rapids machines with 224
logical CPUs.

Inject artifical delays while the bypass mode is switching to guarantee
timely completion.

While at it, move __scx_ops_bypass_lock into scx_ops_bypass() and rename it
to bypass_lock.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Valentin Andrei <vandrei@meta.com>
Reported-by: Patrick Lu <patlu@meta.com>
This commit is contained in:
Tejun Heo 2024-11-05 11:48:11 -10:00
parent f07b806ad8
commit 62dcbab8b0
1 changed files with 52 additions and 3 deletions
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55
kernel/sched/ext.c
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@ -867,8 +867,8 @@ static DEFINE_MUTEX(scx_ops_enable_mutex);
DEFINE_STATIC_KEY_FALSE(__scx_ops_enabled);
DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem);
static atomic_t scx_ops_enable_state_var = ATOMIC_INIT(SCX_OPS_DISABLED);
static atomic_t scx_ops_breather_depth = ATOMIC_INIT(0);
static int scx_ops_bypass_depth;
static DEFINE_RAW_SPINLOCK(__scx_ops_bypass_lock);
static bool scx_ops_init_task_enabled;
static bool scx_switching_all;
DEFINE_STATIC_KEY_FALSE(__scx_switched_all);
@ -2474,10 +2474,47 @@ static struct rq *move_task_between_dsqs(struct task_struct *p, u64 enq_flags,
return dst_rq;
}
/*
* A poorly behaving BPF scheduler can live-lock the system by e.g. incessantly
* banging on the same DSQ on a large NUMA system to the point where switching
* to the bypass mode can take a long time. Inject artifical delays while the
* bypass mode is switching to guarantee timely completion.
*/
static void scx_ops_breather(struct rq *rq)
{
u64 until;
lockdep_assert_rq_held(rq);
if (likely(!atomic_read(&scx_ops_breather_depth)))
return;
raw_spin_rq_unlock(rq);
until = ktime_get_ns() + NSEC_PER_MSEC;
do {
int cnt = 1024;
while (atomic_read(&scx_ops_breather_depth) && --cnt)
cpu_relax();
} while (atomic_read(&scx_ops_breather_depth) &&
time_before64(ktime_get_ns(), until));
raw_spin_rq_lock(rq);
}
static bool consume_dispatch_q(struct rq *rq, struct scx_dispatch_q *dsq)
{
struct task_struct *p;
retry:
/*
* This retry loop can repeatedly race against scx_ops_bypass()
* dequeueing tasks from @dsq trying to put the system into the bypass
* mode. On some multi-socket machines (e.g. 2x Intel 8480c), this can
* live-lock the machine into soft lockups. Give a breather.
*/
scx_ops_breather(rq);
/*
* The caller can't expect to successfully consume a task if the task's
* addition to @dsq isn't guaranteed to be visible somehow. Test
@ -4609,10 +4646,11 @@ bool task_should_scx(struct task_struct *p)
*/
static void scx_ops_bypass(bool bypass)
{
static DEFINE_RAW_SPINLOCK(bypass_lock);
int cpu;
unsigned long flags;
raw_spin_lock_irqsave(&__scx_ops_bypass_lock, flags);
raw_spin_lock_irqsave(&bypass_lock, flags);
if (bypass) {
scx_ops_bypass_depth++;
WARN_ON_ONCE(scx_ops_bypass_depth <= 0);
@ -4625,6 +4663,8 @@ static void scx_ops_bypass(bool bypass)
goto unlock;
}
atomic_inc(&scx_ops_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()
@ -4680,8 +4720,10 @@ static void scx_ops_bypass(bool bypass)
/* resched to restore ticks and idle state */
resched_cpu(cpu);
}
atomic_dec(&scx_ops_breather_depth);
unlock:
raw_spin_unlock_irqrestore(&__scx_ops_bypass_lock, flags);
raw_spin_unlock_irqrestore(&bypass_lock, flags);
}
static void free_exit_info(struct scx_exit_info *ei)
@ -6334,6 +6376,13 @@ static bool scx_dispatch_from_dsq(struct bpf_iter_scx_dsq_kern *kit,
raw_spin_rq_lock(src_rq);
}
/*
* If the BPF scheduler keeps calling this function repeatedly, it can
* cause similar live-lock conditions as consume_dispatch_q(). Insert a
* breather if necessary.
*/
scx_ops_breather(src_rq);
locked_rq = src_rq;
raw_spin_lock(&src_dsq->lock);