Here's the combined patch I was planning on testing but didn't get to
(yet). It implements two things - hard limit on spin duration and
early break if the owner also is spinning on a mutex.
Thanks.
Index: work1/include/linux/sched.h
===================================================================
--- work1.orig/include/linux/sched.h
+++ work1/include/linux/sched.h
@@ -359,7 +359,7 @@ extern signed long schedule_timeout_inte
extern signed long schedule_timeout_killable(signed long timeout);
extern signed long schedule_timeout_uninterruptible(signed long timeout);
asmlinkage void schedule(void);
-extern int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner);
+extern bool mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner);
struct nsproxy;
struct user_namespace;
Index: work1/kernel/sched.c
===================================================================
--- work1.orig/kernel/sched.c
+++ work1/kernel/sched.c
@@ -536,6 +536,10 @@ struct rq {
struct hrtimer hrtick_timer;
#endif
+#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
+ bool spinning_on_mutex;
+#endif
+
#ifdef CONFIG_SCHEDSTATS
/* latency stats */
struct sched_info rq_sched_info;
@@ -4021,16 +4025,44 @@ EXPORT_SYMBOL(schedule);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
/*
- * Look out! "owner" is an entirely speculative pointer
- * access and not reliable.
+ * Maximum mutex owner spin duration in nsecs. Don't spin more then
+ * DEF_TIMESLICE.
+ */
+#define MAX_MUTEX_SPIN_NS (DEF_TIMESLICE * 1000000000LLU / HZ)
+
+/**
+ * mutex_spin_on_owner - optimistic adaptive spinning on locked mutex
+ * @lock: the mutex to spin on
+ * @owner: the current owner (speculative pointer)
+ *
+ * The caller is trying to acquire @lock held by @owner. If @owner is
+ * currently running, it might get unlocked soon and spinning on it can
+ * save the overhead of sleeping and waking up.
+ *
+ * Note that @owner is completely speculative and may be completely
+ * invalid. It should be accessed very carefully.
+ *
+ * Forward progress is guaranteed regardless of locking ordering by never
+ * spinning longer than MAX_MUTEX_SPIN_NS. This is necessary because
+ * mutex_trylock(), which doesn't have to follow the usual locking
+ * ordering, also uses this function.
+ *
+ * CONTEXT:
+ * Preemption disabled.
+ *
+ * RETURNS:
+ * %true if the lock was released and the caller should retry locking.
+ * %false if the caller better go sleeping.
*/
-int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
+bool mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
{
+ unsigned long start;
unsigned int cpu;
struct rq *rq;
+ bool ret = true;
if (!sched_feat(OWNER_SPIN))
- return 0;
+ return false;
#ifdef CONFIG_DEBUG_PAGEALLOC
/*
@@ -4039,7 +4071,7 @@ int mutex_spin_on_owner(struct mutex *lo
* the mutex owner just released it and exited.
*/
if (probe_kernel_address(&owner->cpu, cpu))
- return 0;
+ return false;
#else
cpu = owner->cpu;
#endif
@@ -4049,15 +4081,17 @@ int mutex_spin_on_owner(struct mutex *lo
* the cpu field may no longer be valid.
*/
if (cpu >= nr_cpumask_bits)
- return 0;
+ return false;
/*
* We need to validate that we can do a
* get_cpu() and that we have the percpu area.
*/
if (!cpu_online(cpu))
- return 0;
+ return false;
+ this_rq()->spinning_on_mutex = true;
+ start = local_clock();
rq = cpu_rq(cpu);
for (;;) {
@@ -4070,21 +4104,30 @@ int mutex_spin_on_owner(struct mutex *lo
* we likely have heavy contention. Return 0 to quit
* optimistic spinning and not contend further:
*/
- if (lock->owner)
- return 0;
+ ret = !lock->owner;
break;
}
/*
- * Is that owner really running on that cpu?
+ * Quit spinning if any of the followings is true.
+ *
+ * - The owner isn't running on that cpu.
+ * - The owner also is spinning on a mutex.
+ * - Someone else wants to use this cpu.
+ * - We've been spinning for too long.
*/
- if (task_thread_info(rq->curr) != owner || need_resched())
- return 0;
+ if (task_thread_info(rq->curr) != owner ||
+ rq->spinning_on_mutex || need_resched() ||
+ local_clock() > start + MAX_MUTEX_SPIN_NS) {
+ ret = false;
+ break;
+ }
arch_mutex_cpu_relax();
}
- return 1;
+ this_rq()->spinning_on_mutex = false;
+ return ret;
}
#endif
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