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

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

[Konstantin Khlebnikov]

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)
> --
> 

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

[Sebastian Andrzej Siewior]

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

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

[Ahmed S. Darwish]

Hi Hillf,

For some reason, **all of your posts** from <hdanton@sina.com> do not
appear on lore.kernel.org.

Check, for example, https://lore.kernel.org/lkml/?q=hdanton%40sina.com,
where thread replies are there but not the actual posts.

Just wanted to let you know... Please continue below.

On Wed, Jun 03, 2020 at 10:21:45AM +0200, Sebastian Andrzej Siewior wrote:
> 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;
> >

The patch I've posted makes sure to preserve the existing draining
logic. It only fixes an erroneous usage of seqcount_t latching, plus a
memory barriers bugfix, found by John, and is to be included in v2:

    https://lkml.kernel.org/r/87y2pg9erj.fsf@vostro.fn.ogness.net

On the other hand, you're making the draining operation completely
asynchronous for a number of callers. This is such a huge change, and I
fail to see: 1) any rationale for it in the changelog, 2) whether it's
been verified that call-sites won't be affected.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH