* [PATCH] mm/swap: piggyback lru_add_drain_all() calls
@ 2019-10-04 10:11 Konstantin Khlebnikov
2019-10-04 12:10 ` Matthew Wilcox
0 siblings, 1 reply; 6+ messages in thread
From: Konstantin Khlebnikov @ 2019-10-04 10:11 UTC (permalink / raw)
To: linux-mm, Andrew Morton; +Cc: linux-kernel
This is very slow operation. There is no reason to do it again if somebody
else already drained all per-cpu vectors after we waited for lock.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
mm/swap.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/mm/swap.c b/mm/swap.c
index 38c3fa4308e2..6203918e1316 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -708,9 +708,10 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
*/
void lru_add_drain_all(void)
{
+ static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
static DEFINE_MUTEX(lock);
static struct cpumask has_work;
- int cpu;
+ int cpu, seq;
/*
* Make sure nobody triggers this path before mm_percpu_wq is fully
@@ -719,7 +720,16 @@ void lru_add_drain_all(void)
if (WARN_ON(!mm_percpu_wq))
return;
+ seq = raw_read_seqcount_latch(&seqcount);
+
mutex_lock(&lock);
+
+ /* Piggyback on drain done by somebody else. */
+ if (__read_seqcount_retry(&seqcount, seq))
+ goto done;
+
+ raw_write_seqcount_latch(&seqcount);
+
cpumask_clear(&has_work);
for_each_online_cpu(cpu) {
@@ -740,6 +750,7 @@ 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);
}
#else
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
2019-10-04 10:11 [PATCH] mm/swap: piggyback lru_add_drain_all() calls Konstantin Khlebnikov
@ 2019-10-04 12:10 ` Matthew Wilcox
2019-10-04 12:24 ` Konstantin Khlebnikov
2019-10-04 12:27 ` Michal Hocko
0 siblings, 2 replies; 6+ messages in thread
From: Matthew Wilcox @ 2019-10-04 12:10 UTC (permalink / raw)
To: Konstantin Khlebnikov; +Cc: linux-mm, Andrew Morton, linux-kernel
On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> This is very slow operation. There is no reason to do it again if somebody
> else already drained all per-cpu vectors after we waited for lock.
> + seq = raw_read_seqcount_latch(&seqcount);
> +
> mutex_lock(&lock);
> +
> + /* Piggyback on drain done by somebody else. */
> + if (__read_seqcount_retry(&seqcount, seq))
> + goto done;
> +
> + raw_write_seqcount_latch(&seqcount);
> +
Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
have the same effect?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
2019-10-04 12:10 ` Matthew Wilcox
@ 2019-10-04 12:24 ` Konstantin Khlebnikov
2019-10-04 12:27 ` Michal Hocko
1 sibling, 0 replies; 6+ messages in thread
From: Konstantin Khlebnikov @ 2019-10-04 12:24 UTC (permalink / raw)
To: Matthew Wilcox; +Cc: linux-mm, Andrew Morton, linux-kernel
On 04/10/2019 15.10, Matthew Wilcox wrote:
> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
>> This is very slow operation. There is no reason to do it again if somebody
>> else already drained all per-cpu vectors after we waited for lock.
>> + seq = raw_read_seqcount_latch(&seqcount);
>> +
>> mutex_lock(&lock);
>> +
>> + /* Piggyback on drain done by somebody else. */
>> + if (__read_seqcount_retry(&seqcount, seq))
>> + goto done;
>> +
>> + raw_write_seqcount_latch(&seqcount);
>> +
>
> Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
> have the same effect?
>
No, this is completely different semantics.
Operation could be safely skipped only if somebody else started and
finished drain after current task called this function.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
2019-10-04 12:10 ` Matthew Wilcox
2019-10-04 12:24 ` Konstantin Khlebnikov
@ 2019-10-04 12:27 ` Michal Hocko
2019-10-04 12:32 ` Konstantin Khlebnikov
1 sibling, 1 reply; 6+ messages in thread
From: Michal Hocko @ 2019-10-04 12:27 UTC (permalink / raw)
To: Matthew Wilcox
Cc: Konstantin Khlebnikov, linux-mm, Andrew Morton, linux-kernel
On Fri 04-10-19 05:10:17, Matthew Wilcox wrote:
> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> > This is very slow operation. There is no reason to do it again if somebody
> > else already drained all per-cpu vectors after we waited for lock.
> > + seq = raw_read_seqcount_latch(&seqcount);
> > +
> > mutex_lock(&lock);
> > +
> > + /* Piggyback on drain done by somebody else. */
> > + if (__read_seqcount_retry(&seqcount, seq))
> > + goto done;
> > +
> > + raw_write_seqcount_latch(&seqcount);
> > +
>
> Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
> have the same effect?
Yeah, this makes sense. From correctness point of view it should be ok
because no caller can expect that per-cpu pvecs are empty on return.
This might have some runtime effects that some paths might retry more -
e.g. offlining path drains pcp pvces before migrating the range away, if
there are pages still waiting for a worker to drain them then the
migration would fail and we would retry. But this not a correctness
issue.
--
Michal Hocko
SUSE Labs
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
2019-10-04 12:27 ` Michal Hocko
@ 2019-10-04 12:32 ` Konstantin Khlebnikov
2019-10-04 12:37 ` Michal Hocko
0 siblings, 1 reply; 6+ messages in thread
From: Konstantin Khlebnikov @ 2019-10-04 12:32 UTC (permalink / raw)
To: Michal Hocko, Matthew Wilcox; +Cc: linux-mm, Andrew Morton, linux-kernel
On 04/10/2019 15.27, Michal Hocko wrote:
> On Fri 04-10-19 05:10:17, Matthew Wilcox wrote:
>> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
>>> This is very slow operation. There is no reason to do it again if somebody
>>> else already drained all per-cpu vectors after we waited for lock.
>>> + seq = raw_read_seqcount_latch(&seqcount);
>>> +
>>> mutex_lock(&lock);
>>> +
>>> + /* Piggyback on drain done by somebody else. */
>>> + if (__read_seqcount_retry(&seqcount, seq))
>>> + goto done;
>>> +
>>> + raw_write_seqcount_latch(&seqcount);
>>> +
>>
>> Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
>> have the same effect?
>
> Yeah, this makes sense. From correctness point of view it should be ok
> because no caller can expect that per-cpu pvecs are empty on return.
> This might have some runtime effects that some paths might retry more -
> e.g. offlining path drains pcp pvces before migrating the range away, if
> there are pages still waiting for a worker to drain them then the
> migration would fail and we would retry. But this not a correctness
> issue.
>
Caller might expect that pages added by him before are drained.
Exiting after mutex_trylock() will not guarantee that.
For example POSIX_FADV_DONTNEED uses that.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
2019-10-04 12:32 ` Konstantin Khlebnikov
@ 2019-10-04 12:37 ` Michal Hocko
0 siblings, 0 replies; 6+ messages in thread
From: Michal Hocko @ 2019-10-04 12:37 UTC (permalink / raw)
To: Konstantin Khlebnikov
Cc: Matthew Wilcox, linux-mm, Andrew Morton, linux-kernel
On Fri 04-10-19 15:32:01, Konstantin Khlebnikov wrote:
>
>
> On 04/10/2019 15.27, Michal Hocko wrote:
> > On Fri 04-10-19 05:10:17, Matthew Wilcox wrote:
> > > On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> > > > This is very slow operation. There is no reason to do it again if somebody
> > > > else already drained all per-cpu vectors after we waited for lock.
> > > > + seq = raw_read_seqcount_latch(&seqcount);
> > > > +
> > > > mutex_lock(&lock);
> > > > +
> > > > + /* Piggyback on drain done by somebody else. */
> > > > + if (__read_seqcount_retry(&seqcount, seq))
> > > > + goto done;
> > > > +
> > > > + raw_write_seqcount_latch(&seqcount);
> > > > +
> > >
> > > Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
> > > have the same effect?
> >
> > Yeah, this makes sense. From correctness point of view it should be ok
> > because no caller can expect that per-cpu pvecs are empty on return.
> > This might have some runtime effects that some paths might retry more -
> > e.g. offlining path drains pcp pvces before migrating the range away, if
> > there are pages still waiting for a worker to drain them then the
> > migration would fail and we would retry. But this not a correctness
> > issue.
> >
>
> Caller might expect that pages added by him before are drained.
> Exiting after mutex_trylock() will not guarantee that.
>
> For example POSIX_FADV_DONTNEED uses that.
OK, I was not aware of this case. Please make sure to document that in
the changelog and a comment in the code wouldn't hurt either. It would
certainly explain more thatn "Piggyback on drain done by somebody
else.".
Thanks!
--
Michal Hocko
SUSE Labs
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2019-10-04 12:37 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-10-04 10:11 [PATCH] mm/swap: piggyback lru_add_drain_all() calls Konstantin Khlebnikov
2019-10-04 12:10 ` Matthew Wilcox
2019-10-04 12:24 ` Konstantin Khlebnikov
2019-10-04 12:27 ` Michal Hocko
2019-10-04 12:32 ` Konstantin Khlebnikov
2019-10-04 12:37 ` Michal Hocko
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).