linux-kernel - Re: [External] Re: [PATCH] sched/core: Avoid obvious double update_rq

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Message-ID: <84f61791-8cf2-b955-5d71-1cab15129ab2@bytedance.com>
Date:   Thu, 21 Apr 2022 15:24:33 +0800
From:   Hao Jia <jiahao.os@...edance.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, 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 2022/4/21 Dietmar Eggemann wrote:
> On 20/04/2022 10:29, Hao Jia wrote:
>> On 4/19/22 6:48 PM, Peter Zijlstra wrote:
>>> On Mon, Apr 18, 2022 at 05:09:29PM +0800, Hao Jia wrote:
> 
> [...]
> 
>>> 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.
> 
> 
> [...]
> 
> Maybe something like this:
> 
> -->8--
> 
> From: Dietmar Eggemann <dietmar.eggemann@....com>
> Date: Wed, 20 Apr 2022 11:12:10 +0200
> Subject: [PATCH] sched/core: Clear RQCF_UPDATED in _double_lock_balance() &
>   double_rq_lock()
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
>   kernel/sched/core.c  |  6 +++---
>   kernel/sched/sched.h | 20 ++++++++++++++++----
>   2 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 068c088e9584..f4cfe7eea861 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -610,10 +610,10 @@ 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);
> +	rq_clock_clear_update(rq1, rq2);
>   }
>   #endif
>   
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 58263f90c559..3a77b10d7cc4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2515,6 +2515,16 @@ static inline bool rq_order_less(struct rq *rq1, struct rq *rq2)
>   
>   extern void double_rq_lock(struct rq *rq1, struct rq *rq2);
>   
> +#ifdef CONFIG_SCHED_DEBUG
> +static inline void rq_clock_clear_update(struct rq *rq1, struct rq *rq2)
> +{
> +	rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +	rq2->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
> +}
> +#else
> +static inline void rq_clock_clear_update(struct rq *rq1, struct rq *rq2) {}
> +#endif
> +
>   #ifdef CONFIG_PREEMPTION
>   
>   /*
> @@ -2549,14 +2559,15 @@ static inline int _double_lock_balance(struct rq *this_rq, struct rq *busiest)
>   	__acquires(busiest->lock)
>   	__acquires(this_rq->lock)
>   {
> -	if (__rq_lockp(this_rq) == __rq_lockp(busiest))
> -		return 0;
> -
> -	if (likely(raw_spin_rq_trylock(busiest)))
> +	if (__rq_lockp(this_rq) == __rq_lockp(busiest) ||
> +	    likely(raw_spin_rq_trylock(busiest))) {
> +		rq_clock_clear_update(this_rq, busiest);
>   		return 0;
> +	}
>   
>   	if (rq_order_less(this_rq, busiest)) {
>   		raw_spin_rq_lock_nested(busiest, SINGLE_DEPTH_NESTING);
> +		rq_clock_clear_update(this_rq, busiest);
>   		return 0;
>   	}
>   
> @@ -2650,6 +2661,7 @@ 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 ;) */
> +	rq_clock_clear_update(rq1, rq2);

Thanks for your review.
This is very helpful to me.
If CONFIG_SMP is not enabled, should we just clear the RQCF_UPDATED of 
one of rq1 and q2?

like this:
rq1->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);

Thanks.

>   }
>   
>   /*