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Date: Thu, 6 Apr 2023 18:02:58 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
"Darrick J. Wong" <djwong@...nel.org>,
Christoph Lameter <cl@...ux.com>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Miaohe Lin <linmiaohe@...wei.com>,
David Hildenbrand <david@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
NeilBrown <neilb@...e.de>, Shakeel Butt <shakeelb@...gle.com>,
Michal Hocko <mhocko@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Dave Chinner <david@...morbit.com>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-xfs@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 2/2] mm: vmscan: refactor reclaim_state helpers
On Thu, Apr 6, 2023 at 1:45 PM Peter Xu <peterx@...hat.com> wrote:
>
> Hi, Yosry,
>
> On Wed, Apr 05, 2023 at 06:54:27PM +0000, Yosry Ahmed wrote:
>
> [...]
>
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index c82bd89f90364..049e39202e6ce 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -188,18 +188,6 @@ struct scan_control {
> > */
> > int vm_swappiness = 60;
> >
> > -static void set_task_reclaim_state(struct task_struct *task,
> > - struct reclaim_state *rs)
> > -{
> > - /* Check for an overwrite */
> > - WARN_ON_ONCE(rs && task->reclaim_state);
> > -
> > - /* Check for the nulling of an already-nulled member */
> > - WARN_ON_ONCE(!rs && !task->reclaim_state);
> > -
> > - task->reclaim_state = rs;
> > -}
> > -
> > LIST_HEAD(shrinker_list);
> > DECLARE_RWSEM(shrinker_rwsem);
> >
> > @@ -511,6 +499,59 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> > }
> > #endif
> >
> > +static void set_task_reclaim_state(struct task_struct *task,
> > + struct reclaim_state *rs)
> > +{
> > + /* Check for an overwrite */
> > + WARN_ON_ONCE(rs && task->reclaim_state);
> > +
> > + /* Check for the nulling of an already-nulled member */
> > + WARN_ON_ONCE(!rs && !task->reclaim_state);
> > +
> > + task->reclaim_state = rs;
> > +}
>
> Nit: I just think such movement not necessary while it loses the "git
> blame" information easily.
>
> Instead of moving this here without major benefit, why not just define
> flush_reclaim_state() right after previous set_task_reclaim_state()?
An earlier version did that, but we would have to add a forward
declaration of global_reclaim() (or cgroup_reclaim()), as they are
defined after the previous position of set_task_reclaim_state().
>
> > +
> > +/*
> > + * flush_reclaim_state(): add pages reclaimed outside of LRU-based reclaim to
> > + * scan_control->nr_reclaimed.
> > + */
> > +static void flush_reclaim_state(struct scan_control *sc,
> > + struct reclaim_state *rs)
> > +{
> > + /*
> > + * Currently, reclaim_state->reclaimed includes three types of pages
> > + * freed outside of vmscan:
> > + * (1) Slab pages.
> > + * (2) Clean file pages from pruned inodes.
> > + * (3) XFS freed buffer pages.
> > + *
> > + * For all of these cases, we have no way of finding out whether these
> > + * pages were related to the memcg under reclaim. For example, a freed
> > + * slab page could have had only a single object charged to the memcg
> > + * under reclaim. Also, populated inodes are not on shrinker LRUs
> > + * anymore except on highmem systems.
> > + *
> > + * Instead of over-reporting the reclaimed pages in a memcg reclaim,
> > + * only count such pages in global reclaim. This prevents unnecessary
> > + * retries during memcg charging and false positive from proactive
> > + * reclaim (memory.reclaim).
> > + *
> > + * For uncommon cases were the freed pages were actually significantly
> > + * charged to the memcg under reclaim, and we end up under-reporting, it
> > + * should be fine. The freed pages will be uncharged anyway, even if
> > + * they are not reported properly, and we will be able to make forward
> > + * progress in charging (which is usually in a retry loop).
> > + *
> > + * We can go one step further, and report the uncharged objcg pages in
> > + * memcg reclaim, to make reporting more accurate and reduce
> > + * under-reporting, but it's probably not worth the complexity for now.
> > + */
> > + if (rs && global_reclaim(sc)) {
> > + sc->nr_reclaimed += rs->reclaimed;
> > + rs->reclaimed = 0;
> > + }
> > +}
> > +
> > static long xchg_nr_deferred(struct shrinker *shrinker,
> > struct shrink_control *sc)
> > {
> > @@ -5346,10 +5387,7 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
> > vmpressure(sc->gfp_mask, memcg, false, sc->nr_scanned - scanned,
> > sc->nr_reclaimed - reclaimed);
> >
> > - if (global_reclaim(sc)) {
> > - sc->nr_reclaimed += current->reclaim_state->reclaimed_slab;
> > - current->reclaim_state->reclaimed_slab = 0;
> > - }
> > + flush_reclaim_state(sc, current->reclaim_state);
> >
> > return success ? MEMCG_LRU_YOUNG : 0;
> > }
> > @@ -6474,10 +6512,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >
> > shrink_node_memcgs(pgdat, sc);
> >
> > - if (reclaim_state && global_reclaim(sc)) {
> > - sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> > - reclaim_state->reclaimed_slab = 0;
> > - }
> > + flush_reclaim_state(sc, reclaim_state);
>
> IIUC reclaim_state here still points to current->reclaim_state. Could it
> change at all?
>
> Is it cleaner to make flush_reclaim_state() taking "sc" only if it always
> references current->reclaim_state?
Good point. I think it's always current->reclaim_state.
I think we can make flush_reclaim_state() only take "sc" as an
argument, and remove the "reclaim_state" local variable in
shrink_node() completely.
>
> >
> > /* Record the subtree's reclaim efficiency */
> > if (!sc->proactive)
> > --
> > 2.40.0.348.gf938b09366-goog
> >
>
> --
> Peter Xu
>
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