Re: [PATCH] mm: mglru: Fix soft lockup attributed to scanning folios

[Yu Zhao <yuzhao@google.com>]

On Fri, Mar 08, 2024 at 04:57:08PM +0800, Yafang Shao wrote:
> On Fri, Mar 8, 2024 at 1:06 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Thu,  7 Mar 2024 11:19:52 +0800 Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > > After we enabled mglru on our 384C1536GB production servers, we
> > > encountered frequent soft lockups attributed to scanning folios.
> > >
> > > The soft lockup as follows,
> > >
> > > ...
> > >
> > > There were a total of 22 tasks waiting for this spinlock
> > > (RDI: ffff99d2b6ff9050):
> > >
> > >  crash> foreach RU bt | grep -B 8  queued_spin_lock_slowpath |  grep "RDI: ffff99d2b6ff9050" | wc -l
> > >  22
> >
> > If we're holding the lock for this long then there's a possibility of
> > getting hit by the NMI watchdog also.
> 
> The NMI watchdog is disabled as these servers are KVM guest.
> 
>     kernel.nmi_watchdog = 0
>     kernel.soft_watchdog = 1
> 
> >
> > > Additionally, two other threads were also engaged in scanning folios, one
> > > with 19 waiters and the other with 15 waiters.
> > >
> > > To address this issue under heavy reclaim conditions, we introduced a
> > > hotfix version of the fix, incorporating cond_resched() in scan_folios().
> > > Following the application of this hotfix to our servers, the soft lockup
> > > issue ceased.
> > >
> > > ...
> > >
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -4367,6 +4367,10 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
> > >
> > >                       if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH)
> > >                               break;
> > > +
> > > +                     spin_unlock_irq(&lruvec->lru_lock);
> > > +                     cond_resched();
> > > +                     spin_lock_irq(&lruvec->lru_lock);
> > >               }
> >
> > Presumably wrapping this with `if (need_resched())' will save some work.
> 
> good suggestion.
> 
> >
> > This lock is held for a reason.  I'd like to see an analysis of why
> > this change is safe.
> 
> I believe the key point here is whether we can reduce the scope of
> this lock from:
> 
>   evict_folios
>       spin_lock_irq(&lruvec->lru_lock);
>       scanned = isolate_folios(lruvec, sc, swappiness, &type, &list);
>       scanned += try_to_inc_min_seq(lruvec, swappiness);
>       if (get_nr_gens(lruvec, !swappiness) == MIN_NR_GENS)
>           scanned = 0;
>       spin_unlock_irq(&lruvec->lru_lock);
> 
> to:
> 
>   evict_folios
>       spin_lock_irq(&lruvec->lru_lock);
>       scanned = isolate_folios(lruvec, sc, swappiness, &type, &list);
>       spin_unlock_irq(&lruvec->lru_lock);
> 
>       spin_lock_irq(&lruvec->lru_lock);
>       scanned += try_to_inc_min_seq(lruvec, swappiness);
>       if (get_nr_gens(lruvec, !swappiness) == MIN_NR_GENS)
>           scanned = 0;
>       spin_unlock_irq(&lruvec->lru_lock);
> 
> In isolate_folios(), it merely utilizes the min_seq to retrieve the
> generation without modifying it. If multiple tasks are running
> evict_folios() concurrently, it seems inconsequential whether min_seq
> is incremented by one task or another. I'd appreciate Yu's
> confirmation on this matter.

Hi Yafang,

Thanks for the patch!

Yes, your second analysis is correct -- we can't just drop the lock
as the original patch does because min_seq can be updated in the mean
time. If this happens, the gen value becomes invalid, since it's based
on the expired min_seq:

  sort_folio()
  {
    ..
    gen = lru_gen_from_seq(lrugen->min_seq[type]);
    ..
  }

The following might be a better approach (untested):

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4255619a1a31..6fe53cfa8ef8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4365,7 +4365,8 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
 				skipped_zone += delta;
 			}
 
-			if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH)
+			if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH ||
+			    spin_is_contended(&lruvec->lru_lock))
 				break;
 		}
 
@@ -4375,7 +4376,8 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
 			skipped += skipped_zone;
 		}
 
-		if (!remaining || isolated >= MIN_LRU_BATCH)
+		if (!remaining || isolated >= MIN_LRU_BATCH ||
+		    (scanned && spin_is_contended(&lruvec->lru_lock)))
 			break;
 	}