Re: [RFC PATCH] mm: swap: remove lru drain waiters

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-06-01 22:37:34 [+0800], Hillf Danton wrote:
> 
> After updating the lru drain sequence, new comers avoid waiting for
> the current drainer, because he is flushing works on each online CPU,
> by trying to lock the mutex; the drainer OTOH tries to do works for
> those who fail to acquire the lock by checking the lru drain sequence
> after releasing lock.
> 
> See eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
> for reasons why we can skip waiting for the lock.
> 
> The memory barriers around the sequence and the lock come together
> to remove waiters without their drain works bandoned.
> 
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Signed-off-by: Hillf Danton <hdanton@sina.com>
> ---
> This is inspired by one of the works from Sebastian.

Not me, it was Ahmed.

> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -714,10 +714,11 @@ static void lru_add_drain_per_cpu(struct
>   */
>  void lru_add_drain_all(void)
>  {
> -	static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
> +	static unsigned int lru_drain_seq;
>  	static DEFINE_MUTEX(lock);
>  	static struct cpumask has_work;
> -	int cpu, seq;
> +	int cpu;
> +	unsigned int seq;
>  
>  	/*
>  	 * Make sure nobody triggers this path before mm_percpu_wq is fully
> @@ -726,18 +727,16 @@ void lru_add_drain_all(void)
>  	if (WARN_ON(!mm_percpu_wq))
>  		return;
>  
> -	seq = raw_read_seqcount_latch(&seqcount);
> +	lru_drain_seq++;
> +	smp_mb();
>  
> -	mutex_lock(&lock);
> +more_work:
>  
> -	/*
> -	 * Piggyback on drain started and finished while we waited for lock:
> -	 * all pages pended at the time of our enter were drained from vectors.
> -	 */
> -	if (__read_seqcount_retry(&seqcount, seq))
> -		goto done;
> +	if (!mutex_trylock(&lock))
> +		return;
>  
> -	raw_write_seqcount_latch(&seqcount);
> +	smp_mb();
> +	seq = lru_drain_seq;
>  
>  	cpumask_clear(&has_work);
>  
> @@ -759,8 +758,11 @@ void lru_add_drain_all(void)
>  	for_each_cpu(cpu, &has_work)
>  		flush_work(&per_cpu(lru_add_drain_work, cpu));
>  
> -done:
>  	mutex_unlock(&lock);
> +
> +	smp_mb();
> +	if (seq != lru_drain_seq)
> +		goto more_work;
>  }
>  #else
>  void lru_add_drain_all(void)
> --

Sebastian