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

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 01/06/2020 17.37, 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.

That patch tells nothing about such change in behaviour.

Callers like invalidate_bdev() really need synchronous drain to be sure
that pages have no extra reference from per-cpu vectors.

> 
> 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.
> 
> --- 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)
> --
>