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 <[email protected]>
> Cc: Konstantin Khlebnikov <[email protected]>
> Signed-off-by: Hillf Danton <[email protected]>
> ---
> 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
Hi Hillf,
For some reason, **all of your posts** from <[email protected]> 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 <[email protected]>
> > Cc: Konstantin Khlebnikov <[email protected]>
> > Signed-off-by: Hillf Danton <[email protected]>
> > ---
> > 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/[email protected]
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