Re: [PATCH 27/34] sched_ext: Implement SCX_KICK_WAIT

[Andrea Righi <andrea.righi@canonical.com>]

On Mon, Jul 10, 2023 at 03:13:45PM -1000, Tejun Heo wrote:
...
> +	for_each_cpu_andnot(cpu, this_rq->scx.cpus_to_wait,
> +			    cpumask_of(this_cpu)) {
> +		/*
> +		 * Pairs with smp_store_release() issued by this CPU in
> +		 * scx_notify_pick_next_task() on the resched path.
> +		 *
> +		 * We busy-wait here to guarantee that no other task can be
> +		 * scheduled on our core before the target CPU has entered the
> +		 * resched path.
> +		 */
> +		while (smp_load_acquire(&cpu_rq(cpu)->scx.pnt_seq) == pseqs[cpu])
> +			cpu_relax();
> +	}
> +

...

> +static inline void scx_notify_pick_next_task(struct rq *rq,
> +					     const struct task_struct *p,
> +					     const struct sched_class *active)
> +{
> +#ifdef CONFIG_SMP
> +	if (!scx_enabled())
> +		return;
> +	/*
> +	 * Pairs with the smp_load_acquire() issued by a CPU in
> +	 * kick_cpus_irq_workfn() who is waiting for this CPU to perform a
> +	 * resched.
> +	 */
> +	smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
> +#endif
> +}

We can't use smp_load_acquire()/smp_store_release() with a u64 on
32-bit architectures.

For example, on armhf the build is broken:

In function ‘scx_notify_pick_next_task’,
    inlined from ‘__pick_next_task’ at /<<PKGBUILDDIR>>/kernel/sched/core.c:6106:4,
    inlined from ‘pick_next_task’ at /<<PKGBUILDDIR>>/kernel/sched/core.c:6605:9,
    inlined from ‘__schedule’ at /<<PKGBUILDDIR>>/kernel/sched/core.c:6750:9:
/<<PKGBUILDDIR>>/include/linux/compiler_types.h:397:45: error: call to ‘__compiletime_assert_597’ declared with attribute error: Need native word sized stores/loads for atomicity.
  397 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                             ^
/<<PKGBUILDDIR>>/include/linux/compiler_types.h:378:25: note: in definition of macro ‘__compiletime_assert’
  378 |                         prefix ## suffix();                             \
      |                         ^~~~~~
/<<PKGBUILDDIR>>/include/linux/compiler_types.h:397:9: note: in expansion of macro ‘_compiletime_assert’
  397 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |         ^~~~~~~~~~~~~~~~~~~
/<<PKGBUILDDIR>>/include/linux/compiler_types.h:400:9: note: in expansion of macro ‘compiletime_assert’
  400 |         compiletime_assert(__native_word(t),                            \
      |         ^~~~~~~~~~~~~~~~~~
/<<PKGBUILDDIR>>/include/asm-generic/barrier.h:141:9: note: in expansion of macro ‘compiletime_assert_atomic_type’
  141 |         compiletime_assert_atomic_type(*p);                             \
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/<<PKGBUILDDIR>>/include/asm-generic/barrier.h:172:55: note: in expansion of macro ‘__smp_store_release’
  172 | #define smp_store_release(p, v) do { kcsan_release(); __smp_store_release(p, v); } while (0)
      |                                                       ^~~~~~~~~~~~~~~~~~~
/<<PKGBUILDDIR>>/kernel/sched/ext.h:159:9: note: in expansion of macro ‘smp_store_release’
  159 |         smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);

There's probably a better way to fix this, but for now I've temporarily
solved this using cmpxchg64() - see patch below.

I'm not sure if we already have an equivalent of
smp_store_release_u64/smp_load_acquire_u64(). Otherwise, it may be worth
to add them to a more generic place.

-Andrea

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 051c79fa25f7..5da72b1cf88d 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3667,7 +3667,7 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
 		 * scheduled on our core before the target CPU has entered the
 		 * resched path.
 		 */
-		while (smp_load_acquire(&cpu_rq(cpu)->scx.pnt_seq) == pseqs[cpu])
+		while (smp_load_acquire_u64(&cpu_rq(cpu)->scx.pnt_seq) == pseqs[cpu])
 			cpu_relax();
 	}
 
diff --git a/kernel/sched/ext.h b/kernel/sched/ext.h
index 405037a4e6ce..ef4a24d77d30 100644
--- a/kernel/sched/ext.h
+++ b/kernel/sched/ext.h
@@ -144,6 +144,40 @@ void __scx_notify_pick_next_task(struct rq *rq,
 				 struct task_struct *p,
 				 const struct sched_class *active);
 
+#ifdef CONFIG_64BIT
+static inline u64 smp_load_acquire_u64(u64 *ptr)
+{
+	return smp_load_acquire(ptr);
+}
+
+static inline void smp_store_release_u64(u64 *ptr, u64 val)
+{
+	smp_store_release(ptr, val);
+}
+#else
+static inline u64 smp_load_acquire_u64(u64 *ptr)
+{
+	u64 prev, next;
+
+	do {
+		prev = *ptr;
+		next = prev;
+	} while (cmpxchg64(ptr, prev, next) != prev);
+
+	return prev;
+}
+
+static inline void smp_store_release_u64(u64 *ptr, u64 val)
+{
+	u64 prev, next;
+
+	do {
+		prev = *ptr;
+		next = val;
+	} while (cmpxchg64(ptr, prev, next) != prev);
+}
+#endif
+
 static inline void scx_notify_pick_next_task(struct rq *rq,
 					     struct task_struct *p,
 					     const struct sched_class *active)
@@ -156,7 +190,7 @@ static inline void scx_notify_pick_next_task(struct rq *rq,
 	 * kick_cpus_irq_workfn() who is waiting for this CPU to perform a
 	 * resched.
 	 */
-	smp_store_release(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
+	smp_store_release_u64(&rq->scx.pnt_seq, rq->scx.pnt_seq + 1);
 #endif
 	if (!static_branch_unlikely(&scx_ops_cpu_preempt))
 		return;
-- 
2.40.1