Re: Hard and soft lockups with FIO and LTP runs on a large system

[Bharata B Rao]

On 26-Jul-24 8:56 AM, Zhaoyang Huang wrote:
> On Thu, Jul 25, 2024 at 6:00 PM zhaoyang.huang
> <zhaoyang.huang@xxxxxxxxxx> wrote:
<snip>
> >  From the callstack of lock holder, it is looks like a scability issue rather than a deadlock. Unlike legacy LRU management, there is no throttling mechanism for global reclaim under mglru so far.Could we apply the similar method to throttle the reclaim when it is too aggresive. I am wondering if this patch which is a rough version could help on this?
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2e34de9cd0d4..827036e21f24 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4520,6 +4520,50 @@ static int isolate_folios(struct lruvec *lruvec, struct scan_control *sc, int sw
> >          return scanned;
> >   }
> >
> > +static void lru_gen_throttle(pg_data_t *pgdat, struct scan_control *sc)
> > +{
> > +       struct lruvec *target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
> > +
> > +       if (current_is_kswapd()) {
> > +               if (sc->nr.writeback && sc->nr.writeback == sc->nr.taken)
> > +                       set_bit(PGDAT_WRITEBACK, &pgdat->flags);
> > +
> > +               /* Allow kswapd to start writing pages during reclaim.*/
> > +               if (sc->nr.unqueued_dirty == sc->nr.file_taken)
> > +                       set_bit(PGDAT_DIRTY, &pgdat->flags);
> > +
> > +               if (sc->nr.immediate)
> > +                       reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
> > +       }
> > +
> > +       /*
> > +        * Tag a node/memcg as congested if all the dirty pages were marked
> > +        * for writeback and immediate reclaim (counted in nr.congested).
> > +        *
> > +        * Legacy memcg will stall in page writeback so avoid forcibly
> > +        * stalling in reclaim_throttle().
> > +        */
> > +       if (sc->nr.dirty && (sc->nr.dirty / 2 < sc->nr.congested)) {
> > +               if (cgroup_reclaim(sc) && writeback_throttling_sane(sc))
> > +                       set_bit(LRUVEC_CGROUP_CONGESTED, &target_lruvec->flags);
> > +
> > +               if (current_is_kswapd())
> > +                       set_bit(LRUVEC_NODE_CONGESTED, &target_lruvec->flags);
> > +       }
> > +
> > +       /*
> > +        * Stall direct reclaim for IO completions if the lruvec is
> > +        * node is congested. Allow kswapd to continue until it
> > +        * starts encountering unqueued dirty pages or cycling through
> > +        * the LRU too quickly.
> > +        */
> > +       if (!current_is_kswapd() && current_may_throttle() &&
> > +           !sc->hibernation_mode &&
> > +           (test_bit(LRUVEC_CGROUP_CONGESTED, &target_lruvec->flags) ||
> > +            test_bit(LRUVEC_NODE_CONGESTED, &target_lruvec->flags)))
> > +               reclaim_throttle(pgdat, VMSCAN_THROTTLE_CONGESTED);
> > +}
> > +
> >   static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swappiness)
> >   {
> >          int type;
> > @@ -4552,6 +4596,16 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> >   retry:
> >          reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false);
> >          sc->nr_reclaimed += reclaimed;
> > +       sc->nr.dirty += stat.nr_dirty;
> > +       sc->nr.congested += stat.nr_congested;
> > +       sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> > +       sc->nr.writeback += stat.nr_writeback;
> > +       sc->nr.immediate += stat.nr_immediate;
> > +       sc->nr.taken += scanned;
> > +
> > +       if (type)
> > +               sc->nr.file_taken += scanned;
> > +
> >          trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> >                          scanned, reclaimed, &stat, sc->priority,
> >                          type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
> > @@ -5908,6 +5962,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >
> >          if (lru_gen_enabled() && root_reclaim(sc)) {
> >                  lru_gen_shrink_node(pgdat, sc);
> > +               lru_gen_throttle(pgdat, sc);
> >                  return;
> >          }
> Hi Bharata,
> This patch arised from a regression Android test case failure which
> allocated 1GB virtual memory by each over 8 threads on an 5.5GB RAM
> system. This test could pass on legacy LRU management while failing
> under MGLRU as a watchdog monitor detected abnormal system-wide
> schedule status(watchdog can't be scheduled within 60 seconds). This
> patch with a slight change as below got passed in the test whereas has
> not been investigated deeply for how it was done. Theoretically, this
> patch enrolled the similar reclaim throttle mechanism as legacy do
> which could reduce the contention of lruvec->lru_lock. I think this
> patch is quite naive for now， but I am hoping it could help you as
> your case seems like a scability issue of memory pressure rather than
> a deadlock issue. Thank you!
>
> the change of the applied version(try to throttle the reclaim before
> instead of after)
>           if (lru_gen_enabled() && root_reclaim(sc)) {
>   +               lru_gen_throttle(pgdat, sc);
>                   lru_gen_shrink_node(pgdat, sc);
>   -               lru_gen_throttle(pgdat, sc);
>                   return;
>           }

Thanks Zhaoyang Huang for the patch, will give this a test and report back.

Regards,
Bharata.