[PATCH v10 06/14] mm: multi-gen LRU: minimal implementation

[Barry Song]

On Tue, Apr 19, 2022 at 12:54 PM Yu Zhao <yuzhao at google.com> wrote:
>
> On Mon, Apr 18, 2022 at 3:58 AM Barry Song <21cnbao at gmail.com> wrote:
> >
> > On Thu, Apr 7, 2022 at 3:16 PM Yu Zhao <yuzhao at google.com> wrote:
> > >
> > > To avoid confusion, the terms "promotion" and "demotion" will be
> > > applied to the multi-gen LRU, as a new convention; the terms
> > > "activation" and "deactivation" will be applied to the active/inactive
> > > LRU, as usual.
> > >
> > > The aging produces young generations. Given an lruvec, it increments
> > > max_seq when max_seq-min_seq+1 approaches MIN_NR_GENS. The aging
> > > promotes hot pages to the youngest generation when it finds them
> > > accessed through page tables; the demotion of cold pages happens
> > > consequently when it increments max_seq. The aging has the complexity
> > > O(nr_hot_pages), since it is only interested in hot pages. Promotion
> > > in the aging path does not require any LRU list operations, only the
> > > updates of the gen counter and lrugen->nr_pages[]; demotion, unless as
> > > the result of the increment of max_seq, requires LRU list operations,
> > > e.g., lru_deactivate_fn().
> > >
> > > The eviction consumes old generations. Given an lruvec, it increments
> > > min_seq when the lists indexed by min_seq%MAX_NR_GENS become empty. A
> > > feedback loop modeled after the PID controller monitors refaults over
> > > anon and file types and decides which type to evict when both types
> > > are available from the same generation.
> > >
> > > Each generation is divided into multiple tiers. Tiers represent
> > > different ranges of numbers of accesses through file descriptors. A
> > > page accessed N times through file descriptors is in tier
> > > order_base_2(N). Tiers do not have dedicated lrugen->lists[], only
> > > bits in folio->flags. In contrast to moving across generations, which
> > > requires the LRU lock, moving across tiers only involves operations on
> > > folio->flags. The feedback loop also monitors refaults over all tiers
> > > and decides when to protect pages in which tiers (N>1), using the
> > > first tier (N=0,1) as a baseline. The first tier contains single-use
> > > unmapped clean pages, which are most likely the best choices. The
> > > eviction moves a page to the next generation, i.e., min_seq+1, if the
> > > feedback loop decides so. This approach has the following advantages:
> > > 1. It removes the cost of activation in the buffered access path by
> > >    inferring whether pages accessed multiple times through file
> > >    descriptors are statistically hot and thus worth protecting in the
> > >    eviction path.
> > > 2. It takes pages accessed through page tables into account and avoids
> > >    overprotecting pages accessed multiple times through file
> > >    descriptors. (Pages accessed through page tables are in the first
> > >    tier, since N=0.)
> > > 3. More tiers provide better protection for pages accessed more than
> > >    twice through file descriptors, when under heavy buffered I/O
> > >    workloads.
> > >
> >
> > Hi Yu,
> > As I told you before,  I tried to change the current LRU (not MGLRU) by only
> > promoting unmapped file pages to the head of the inactive head rather than
> > the active head on its second access:
> > https://lore.kernel.org/lkml/CAGsJ_4y=TkCGoWWtWSAptW4RDFUEBeYXwfwu=fUFvV4Sa4VA4A@mail.gmail.com/
> > I have already seen some very good results by the decease of cpu consumption of
> > kswapd and direct reclamation in the testing.
>
> Glad to hear. I suspected you'd see some good results with that change :)
>
> > in mglru, it seems "twice" isn't a concern at all, one unmapped file
> > page accessed
> > twice has no much difference with those ones which are accessed once as you
> > only begin to increase refs from the third time:
>
> refs are *additional* accesses:
> PG_referenced: N=1
> PG_referenced+PG_workingset: N=2
> PG_referenced+PG_workingset+refs: N=3,4,5
>
> When N=2, order_base_2(N)=1. So pages accessed twice are in the second
> tier. Therefore they are "different".
>
> More details [1]:
>
> +/*
> + * Each generation is divided into multiple tiers. Tiers represent different
> + * ranges of numbers of accesses through file descriptors. A page accessed N
> + * times through file descriptors is in tier order_base_2(N). A page in the
> + * first tier (N=0,1) is marked by PG_referenced unless it was faulted in
> + * though page tables or read ahead. A page in any other tier (N>1) is marked
> + * by PG_referenced and PG_workingset.
> + *
> + * In contrast to moving across generations which requires the LRU lock, moving
> + * across tiers only requires operations on folio->flags and therefore has a
> + * negligible cost in the buffered access path. In the eviction path,
> + * comparisons of refaulted/(evicted+protected) from the first tier and the
> + * rest infer whether pages accessed multiple times through file descriptors
> + * are statistically hot and thus worth protecting.
> + *
> + * MAX_NR_TIERS is set to 4 so that the multi-gen LRU can support twice of the
> + * categories of the active/inactive LRU when keeping track of accesses through
> + * file descriptors. It requires MAX_NR_TIERS-2 additional bits in
> folio->flags.
> + */
> +#define MAX_NR_TIERS 4U
>
> [1] https://lore.kernel.org/linux-mm/20220407031525.2368067-7-yuzhao@google.com/
>
> > +static void folio_inc_refs(struct folio *folio)
> > +{
> > +       unsigned long refs;
> > +       unsigned long old_flags, new_flags;
> > +
> > +       if (folio_test_unevictable(folio))
> > +               return;
> > +
> > +       /* see the comment on MAX_NR_TIERS */
> > +       do {
> > +               new_flags = old_flags = READ_ONCE(folio->flags);
> > +
> > +               if (!(new_flags & BIT(PG_referenced))) {
> > +                       new_flags |= BIT(PG_referenced);
> > +                       continue;
> > +               }
> > +
> > +               if (!(new_flags & BIT(PG_workingset))) {
> > +                       new_flags |= BIT(PG_workingset);
> > +                       continue;
> > +               }
> > +
> > +               refs = new_flags & LRU_REFS_MASK;
> > +               refs = min(refs + BIT(LRU_REFS_PGOFF), LRU_REFS_MASK);
> > +
> > +               new_flags &= ~LRU_REFS_MASK;
> > +               new_flags |= refs;
> > +       } while (new_flags != old_flags &&
> > +                cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);
> > +}
> >
> > So my question is what makes you so confident that twice doesn't need
> > any special treatment while the vanilla kernel is upgrading this kind of page
> > to the head of the active instead? I am asking this because I am considering
> > reclaiming unmapped file pages which are only accessed twice when they
> > get to the tail of the inactive list.
>
> Per above, pages accessed twice are in their own tier. Hope this clarifies it.

Yep, I found the trick here , "+1" is magic behind the code, haha.

+static int folio_lru_tier(struct folio *folio)
+{
+    int refs;
+    unsigned long flags = READ_ONCE(folio->flags);
+
+    refs = (flags & LRU_REFS_FLAGS) == LRU_REFS_FLAGS ?
+          ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) + 1 : 0;
+
+    return lru_tier_from_refs(refs);
+}
+

TBH, this might need some comments, otherwise, it is easy to misunderstand
we are beginning to have protection from 3rd access :-)

Thanks
barry