linux-kernel - Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

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Date:   Wed, 28 Apr 2021 14:02:36 +0800
From:   Aubrey Li <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>, joel@...lfernandes.org,
        chris.hyser@...cle.com, joshdon@...gle.com, mingo@...nel.org,
        vincent.guittot@...aro.org, valentin.schneider@....com,
        mgorman@...e.de
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

On 4/22/21 8:05 PM, Peter Zijlstra wrote:
> When switching on core-sched, CPUs need to agree which lock to use for
> their RQ.
> 
> The new rule will be that rq->core_enabled will be toggled while
> holding all rq->__locks that belong to a core. This means we need to
> double check the rq->core_enabled value after each lock acquire and
> retry if it changed.
> 
> This also has implications for those sites that take multiple RQ
> locks, they need to be careful that the second lock doesn't end up
> being the first lock.
> 
> Verify the lock pointer after acquiring the first lock, because if
> they're on the same core, holding any of the rq->__lock instances will
> pin the core state.
> 
> While there, change the rq->__lock order to CPU number, instead of rq
> address, this greatly simplifies the next patch.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/sched/core.c  |   48 ++++++++++++++++++++++++++++++++++++++++++++++--
>  kernel/sched/sched.h |   41 +++++++++++------------------------------
>  2 files changed, 57 insertions(+), 32 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -186,12 +186,37 @@ int sysctl_sched_rt_runtime = 950000;
>  
>  void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
>  {
> -	raw_spin_lock_nested(rq_lockp(rq), subclass);
> +	raw_spinlock_t *lock;
> +
> +	if (sched_core_disabled()) {
> +		raw_spin_lock_nested(&rq->__lock, subclass);
> +		return;
> +	}
> +
> +	for (;;) {
> +		lock = rq_lockp(rq);
> +		raw_spin_lock_nested(lock, subclass);
> +		if (likely(lock == rq_lockp(rq)))
> +			return;
> +		raw_spin_unlock(lock);
> +	}
>  }
>  
>  bool raw_spin_rq_trylock(struct rq *rq)
>  {
> -	return raw_spin_trylock(rq_lockp(rq));
> +	raw_spinlock_t *lock;
> +	bool ret;
> +
> +	if (sched_core_disabled())
> +		return raw_spin_trylock(&rq->__lock);
> +
> +	for (;;) {
> +		lock = rq_lockp(rq);
> +		ret = raw_spin_trylock(lock);
> +		if (!ret || (likely(lock == rq_lockp(rq))))
> +			return ret;
> +		raw_spin_unlock(lock);
> +	}
>  }
>  
>  void raw_spin_rq_unlock(struct rq *rq)
> @@ -199,6 +224,25 @@ void raw_spin_rq_unlock(struct rq *rq)
>  	raw_spin_unlock(rq_lockp(rq));
>  }
>  
> +#ifdef CONFIG_SMP
> +/*
> + * double_rq_lock - safely lock two runqueues
> + */
> +void double_rq_lock(struct rq *rq1, struct rq *rq2)
> +{
> +	lockdep_assert_irqs_disabled();
> +
> +	if (rq1->cpu > rq2->cpu)
> +		swap(rq1, rq2);

I'm not sure why swap rq here instead of rq lock? This swaps dst rq
and src rq and causes the subsequent logic wrong at least in try_steal_cookie().

Thanks,
-Aubrey