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[PATCH v4] sched/core: Adapt WARN_DOUBLE_CLOCK machinery for core-sched

Hao Jia posted 1 patch 2 years, 5 months ago 
kernel/sched/core.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
[PATCH v4] sched/core: Adapt WARN_DOUBLE_CLOCK machinery for core-sched
Posted by Hao Jia 2 years, 5 months ago 
When sched_core_enabled(), we sometimes need to call update_rq_clock()
to update the rq clock of sibling CPUs on the same core, before that we
need to clear RQCF_UPDATED of rq->clock_update_flags to avoid the
WARN_DOUBLE_CLOCK warning. Because at this time the rq->clock_update_flags
of sibling CPUs may be RQCF_UPDATED. If sched_core_enabled(), we will get
a core-wide rq->lock, so at this point we can safely clear RQCF_UPDATED of
rq->clock_update_flags of all CPUs on this core to avoid the
WARN_DOUBLE_CLOCK warning.

We sometimes use rq_pin_lock() and raw_spin_rq_lock() separately,
For example newidle_balance() and _double_lock_balance(). We will
temporarily give up core-wide rq->lock, and then use raw_spin_rq_lock()
to reacquire core-wide rq->lock without rq_pin_lock(), so We can not
clear RQCF_UPDATED of rq->clock_update_flags of other cpus on the
same core in rq_pin_lock().

Steps to reproduce:
1. Enable CONFIG_SCHED_DEBUG and CONFIG_SCHED_CORE when compiling
   the kernel
2. echo 1 > /sys/kernel/debug/clear_warn_once
   echo "WARN_DOUBLE_CLOCK" > /sys/kernel/debug/sched/features
3. Run the linux/tools/testing/selftests/sched/cs_prctl_test test

Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Reviewed-by: Phil Auld <pauld@redhat.com>
---
v3->v4:
 - Replace "core wide" with "core-wide" everywhere.
 - Add "Reviewed-by: Phil Auld <pauld@redhat.com>".
 [v3] https://lore.kernel.org/all/20230330035827.16937-1-jiahao.os@bytedance.com

v2->v3:
 - Modify the function name to sched_core_clear_rqcf_updated,
   and add function comments.
 - Modify commit information.
 [v2] https://lore.kernel.org/all/20230215073927.97802-1-jiahao.os@bytedance.com

v1->v2:
 - Adapt WARN_DOUBLE_CLOCK machinery for core-sched instead of clearing
   WARN_DOUBLE_CLOCK warning one by one.
 - Modify commit information
 [v1] https://lore.kernel.org/all/20221206070550.31763-1-jiahao.os@bytedance.com

 kernel/sched/core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..c6e2c79152ef 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -429,11 +429,32 @@ void sched_core_put(void)
 		schedule_work(&_work);
 }
 
+/*
+ * Now, we have obtained a core-wide rq->lock, then we need to clear
+ * RQCF_UPDATED of rq->clock_update_flags of the sibiling CPU
+ * on this core to avoid the WARN_DOUBLE_CLOCK warning.
+ */
+static inline void sched_core_clear_rqcf_updated(struct rq *rq)
+{
+#ifdef CONFIG_SCHED_DEBUG
+	const struct cpumask *smt_mask;
+	int i;
+
+	if (rq->core_enabled) {
+		smt_mask = cpu_smt_mask(rq->cpu);
+		for_each_cpu(i, smt_mask) {
+			if (rq->cpu != i)
+				cpu_rq(i)->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+		}
+	}
+#endif
+}
 #else /* !CONFIG_SCHED_CORE */
 
 static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
 static inline void
 sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
+static inline void sched_core_clear_rqcf_updated(struct rq *rq) { }
 
 #endif /* CONFIG_SCHED_CORE */
 
@@ -548,6 +569,7 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
 		if (likely(lock == __rq_lockp(rq))) {
 			/* preempt_count *MUST* be > 1 */
 			preempt_enable_no_resched();
+			sched_core_clear_rqcf_updated(rq);
 			return;
 		}
 		raw_spin_unlock(lock);
-- 
2.37.0
Re: [PATCH v4] sched/core: Adapt WARN_DOUBLE_CLOCK machinery for core-sched
Posted by Peter Zijlstra 2 years, 4 months ago 
On Thu, Apr 06, 2023 at 02:44:15PM +0800, Hao Jia wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0d18c3969f90..c6e2c79152ef 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -429,11 +429,32 @@ void sched_core_put(void)
>  		schedule_work(&_work);
>  }
>  
> +/*
> + * Now, we have obtained a core-wide rq->lock, then we need to clear
> + * RQCF_UPDATED of rq->clock_update_flags of the sibiling CPU
> + * on this core to avoid the WARN_DOUBLE_CLOCK warning.
> + */
> +static inline void sched_core_clear_rqcf_updated(struct rq *rq)
> +{
> +#ifdef CONFIG_SCHED_DEBUG
> +	const struct cpumask *smt_mask;
> +	int i;
> +
> +	if (rq->core_enabled) {
> +		smt_mask = cpu_smt_mask(rq->cpu);
> +		for_each_cpu(i, smt_mask) {
> +			if (rq->cpu != i)
> +				cpu_rq(i)->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +		}
> +	}
> +#endif
> +}
>  #else /* !CONFIG_SCHED_CORE */
>  
>  static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
>  static inline void
>  sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
> +static inline void sched_core_clear_rqcf_updated(struct rq *rq) { }
>  
>  #endif /* CONFIG_SCHED_CORE */
>  
> @@ -548,6 +569,7 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
>  		if (likely(lock == __rq_lockp(rq))) {
>  			/* preempt_count *MUST* be > 1 */
>  			preempt_enable_no_resched();
> +			sched_core_clear_rqcf_updated(rq);
>  			return;
>  		}
>  		raw_spin_unlock(lock);

This still looks absolutely wrong. The whole RQCF thing is a pin action.
Re: [External] Re: [PATCH v4] sched/core: Adapt WARN_DOUBLE_CLOCK machinery for core-sched
Posted by Hao Jia 2 years, 4 months ago 

On 2023/5/4 Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 02:44:15PM +0800, Hao Jia wrote:
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 0d18c3969f90..c6e2c79152ef 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -429,11 +429,32 @@ void sched_core_put(void)
>>   		schedule_work(&_work);
>>   }
>>   
>> +/*
>> + * Now, we have obtained a core-wide rq->lock, then we need to clear
>> + * RQCF_UPDATED of rq->clock_update_flags of the sibiling CPU
>> + * on this core to avoid the WARN_DOUBLE_CLOCK warning.
>> + */
>> +static inline void sched_core_clear_rqcf_updated(struct rq *rq)
>> +{
>> +#ifdef CONFIG_SCHED_DEBUG
>> +	const struct cpumask *smt_mask;
>> +	int i;
>> +
>> +	if (rq->core_enabled) {
>> +		smt_mask = cpu_smt_mask(rq->cpu);
>> +		for_each_cpu(i, smt_mask) {
>> +			if (rq->cpu != i)
>> +				cpu_rq(i)->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
>> +		}
>> +	}
>> +#endif
>> +}
>>   #else /* !CONFIG_SCHED_CORE */
>>   
>>   static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
>>   static inline void
>>   sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
>> +static inline void sched_core_clear_rqcf_updated(struct rq *rq) { }
>>   
>>   #endif /* CONFIG_SCHED_CORE */
>>   
>> @@ -548,6 +569,7 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
>>   		if (likely(lock == __rq_lockp(rq))) {
>>   			/* preempt_count *MUST* be > 1 */
>>   			preempt_enable_no_resched();
>> +			sched_core_clear_rqcf_updated(rq);
>>   			return;
>>   		}
>>   		raw_spin_unlock(lock);
> 
> This still looks absolutely wrong. The whole RQCF thing is a pin action.

Do you think it is better for us to extend rq_pin_lock() to clean RQCF 
updated than to do it in raw_spin_rq_lock_nested()?

Before doing this, we need to solve the situation where rq_pin_lock() 
and raw_spin_rq_lock() are used separately.

Any suggestion will be very helpful for me.

Thanks,
Hao

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