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Date: Wed, 20 Apr 2022 16:29:24 +0800
From: Hao Jia <jiahao.os@...edance.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH] sched/core: Avoid obvious double
update_rq_clock warning
On 4/19/22 6:48 PM, Peter Zijlstra wrote:
> On Mon, Apr 18, 2022 at 05:09:29PM +0800, Hao Jia wrote:
>> When we use raw_spin_rq_lock to acquire the rq lock and have to
>> update the rq clock while holding the lock, the kernel may issue
>> a WARN_DOUBLE_CLOCK warning.
>>
>> Since we directly use raw_spin_rq_lock to acquire rq lock instead of
>> rq_lock, there is no corresponding change to rq->clock_update_flags.
>> In particular, we have obtained the rq lock of other cores,
>> the core rq->clock_update_flags may be RQCF_UPDATED at this time, and
>> then calling update_rq_clock will trigger the WARN_DOUBLE_CLOCK warning.
>
>> Signed-off-by: Hao Jia <jiahao.os@...edance.com>
>> ---
>> kernel/sched/deadline.c | 18 +++++++++++-------
>> kernel/sched/rt.c | 20 ++++++++++++++++++--
>
> Very good for keeping them in sync.
>
>> 2 files changed, 29 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index fb4255ae0b2c..9207b978cc43 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>
>> @@ -2317,16 +2318,14 @@ static int push_dl_task(struct rq *rq)
>> goto retry;
>> }
>>
>> + rq_pin_lock(rq, &srf);
>> + rq_pin_lock(later_rq, &drf);
>> deactivate_task(rq, next_task, 0);
>> set_task_cpu(next_task, later_rq->cpu);
>> -
>> - /*
>> - * Update the later_rq clock here, because the clock is used
>> - * by the cpufreq_update_util() inside __add_running_bw().
>> - */
>> - update_rq_clock(later_rq);
>> - activate_task(later_rq, next_task, ENQUEUE_NOCLOCK);
>> + activate_task(later_rq, next_task, 0);
>> ret = 1;
>> + rq_unpin_lock(rq, &srf);
>> + rq_unpin_lock(later_rq, &drf);
>>
>> resched_curr(later_rq);
>>
>
>> @@ -2413,11 +2413,15 @@ static void pull_dl_task(struct rq *this_rq)
>> if (is_migration_disabled(p)) {
>> push_task = get_push_task(src_rq);
>> } else {
>> + rq_pin_lock(this_rq, &this_rf);
>> + rq_pin_lock(src_rq, &src_rf);
>> deactivate_task(src_rq, p, 0);
>> set_task_cpu(p, this_cpu);
>> activate_task(this_rq, p, 0);
>> dmin = p->dl.deadline;
>> resched = true;
>> + rq_unpin_lock(this_rq, &this_rf);
>> + rq_unpin_lock(src_rq, &src_rf);
>> }
>>
>> /* Is there any other task even earlier? */
>
> I'm really not sure about this part though. This is a bit of a mess. The
> balancer doesn't really need the pinning stuff. I realize you did that
> because we got the clock annotation mixed up with that, but urgh.
>
> Basically we want double_rq_lock() / double_lock_balance() to clear
> RQCF_UPDATED, right? Perhaps do that directly?
>
> (maybe with an inline helper and a wee comment?)
>
> The only immediate problem with this would appear to be that
> _double_rq_lock() behaves differently when it returns 0. Not sure that
> matters.
>
> Hmm?
Thanks for your review comments.
As you have prompted, the WARN_DOUBLE_CLOCK warning is still triggered
when _double_rq_lock() returns 0.
Please review the solution below, and based on your review, I will
submit the v2 patch as soon as possible.
Thanks.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 51efaabac3e4..b73fe46cd6c7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -610,10 +610,13 @@ void double_rq_lock(struct rq *rq1, struct rq *rq2)
swap(rq1, rq2);
raw_spin_rq_lock(rq1);
- if (__rq_lockp(rq1) == __rq_lockp(rq2))
- return;
+ if (__rq_lockp(rq1) != __rq_lockp(rq2))
+ raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
- raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
+#ifdef CONFIG_SCHED_DEBUG
+ rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+ rq2->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+#endif
}
#endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8dccb34eb190..9fe506a6b7b4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2544,20 +2544,25 @@ static inline int _double_lock_balance(struct rq
*this_rq, struct rq *busiest)
__acquires(this_rq->lock)
{
if (__rq_lockp(this_rq) == __rq_lockp(busiest))
- return 0;
+ goto out;
if (likely(raw_spin_rq_trylock(busiest)))
- return 0;
+ goto out;
if (rq_order_less(this_rq, busiest)) {
raw_spin_rq_lock_nested(busiest, SINGLE_DEPTH_NESTING);
- return 0;
+ goto out;
+ } else {
+ raw_spin_rq_unlock(this_rq);
+ double_rq_lock(this_rq, busiest);
+ return 1;
}
-
- raw_spin_rq_unlock(this_rq);
- double_rq_lock(this_rq, busiest);
-
- return 1;
+out:
+#ifdef CONFIG_SCHED_DEBUG
+ this_rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+ busiest->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+#endif
+ return 0;
}
#endif /* CONFIG_PREEMPTION */
@@ -2644,6 +2649,9 @@ static inline void double_rq_lock(struct rq *rq1,
struct rq *rq2)
BUG_ON(rq1 != rq2);
raw_spin_rq_lock(rq1);
__acquire(rq2->lock); /* Fake it out ;) */
+#ifdef CONFIG_SCHED_DEBUG
+ rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
+#endif
}
>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f259621f4c93..be4baec84430 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -610,10 +610,13 @@ void double_rq_lock(struct rq *rq1, struct rq *rq2)
> swap(rq1, rq2);
>
> raw_spin_rq_lock(rq1);
> - if (__rq_lockp(rq1) == __rq_lockp(rq2))
> - return;
> + if (__rq_lockp(rq1) != __rq_lockp(rq2))
> + raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
>
> - raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
> +#ifdef CONFIG_SCHED_DEBUG
> + rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> + rq2->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +#endif
> }
> #endif
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8dccb34eb190..3ca8dd5ca17c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2644,6 +2644,10 @@ static inline void double_rq_lock(struct rq *rq1, struct rq *rq2)
> BUG_ON(rq1 != rq2);
> raw_spin_rq_lock(rq1);
> __acquire(rq2->lock); /* Fake it out ;) */
> +#ifdef CONFIG_SCHED_DEBUG
> + rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> + rq2->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +#endif
> }
>
> /*
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