lkml.org 
[lkml]   [2022]   [Apr]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v10 05/14] mm: multi-gen LRU: groundwork
    On Wed,  6 Apr 2022 21:15:17 -0600 Yu Zhao <yuzhao@google.com> wrote:

    > Evictable pages are divided into multiple generations for each lruvec.
    > The youngest generation number is stored in lrugen->max_seq for both
    > anon and file types as they are aged on an equal footing. The oldest
    > generation numbers are stored in lrugen->min_seq[] separately for anon
    > and file types as clean file pages can be evicted regardless of swap
    > constraints. These three variables are monotonically increasing.
    >
    > ...
    >
    > +static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)

    There's a lot of function inlining here. Fortunately the compiler will
    ignore it all, because some of it looks wrong. Please review (and
    remeasure!). If inlining is reqlly justified, use __always_inline, and
    document the reasons for doing so.

    > +{
    > + int gen;
    > + unsigned long old_flags, new_flags;
    > +
    > + do {
    > + new_flags = old_flags = READ_ONCE(folio->flags);
    > + if (!(new_flags & LRU_GEN_MASK))
    > + return false;
    > +
    > + VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
    > + VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
    > +
    > + gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
    > +
    > + new_flags &= ~LRU_GEN_MASK;
    > + /* for shrink_page_list() */
    > + if (reclaiming)
    > + new_flags &= ~(BIT(PG_referenced) | BIT(PG_reclaim));
    > + else if (lru_gen_is_active(lruvec, gen))
    > + new_flags |= BIT(PG_active);
    > + } while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);

    Clearly the cmpxchg loop is handling races against a concurrent
    updater. But it's unclear who that updater is, what are the dynamics
    here and why the designer didn't use, say, spin_lock(). The way to
    clarify such thigs is with code comments!

    >
    > +#endif /* !__GENERATING_BOUNDS_H */
    > +
    > +/*
    > + * Evictable pages are divided into multiple generations. The youngest and the
    > + * oldest generation numbers, max_seq and min_seq, are monotonically increasing.
    > + * They form a sliding window of a variable size [MIN_NR_GENS, MAX_NR_GENS]. An
    > + * offset within MAX_NR_GENS, gen, indexes the LRU list of the corresponding

    The "within MAX_NR_GENS, gen," text here is unclear?

    > + * generation. The gen counter in folio->flags stores gen+1 while a page is on
    > + * one of lrugen->lists[]. Otherwise it stores 0.
    > + *
    > + * A page is added to the youngest generation on faulting. The aging needs to
    > + * check the accessed bit at least twice before handing this page over to the
    > + * eviction. The first check takes care of the accessed bit set on the initial
    > + * fault; the second check makes sure this page hasn't been used since then.
    > + * This process, AKA second chance, requires a minimum of two generations,
    > + * hence MIN_NR_GENS. And to maintain ABI compatibility with the active/inactive

    Where is the ABI compatibility issue? Is it in some way in which the
    legacy LRU is presented to userspace?

    > + * LRU, these two generations are considered active; the rest of generations, if
    > + * they exist, are considered inactive. See lru_gen_is_active(). PG_active is
    > + * always cleared while a page is on one of lrugen->lists[] so that the aging
    > + * needs not to worry about it. And it's set again when a page considered active
    > + * is isolated for non-reclaiming purposes, e.g., migration. See
    > + * lru_gen_add_folio() and lru_gen_del_folio().
    > + *
    > + * MAX_NR_GENS is set to 4 so that the multi-gen LRU can support twice of the

    "twice the number of"?
    > + * categories of the active/inactive LRU when keeping track of accesses through
    > + * page tables. It requires order_base_2(MAX_NR_GENS+1) bits in folio->flags.
    > + */

    Helpful comment, overall.

    >
    > ...
    >
    > --- a/mm/Kconfig
    > +++ b/mm/Kconfig
    > @@ -909,6 +909,14 @@ config ANON_VMA_NAME
    > area from being merged with adjacent virtual memory areas due to the
    > difference in their name.
    >
    > +config LRU_GEN
    > + bool "Multi-Gen LRU"
    > + depends on MMU
    > + # the following options can use up the spare bits in page flags
    > + depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
    > + help
    > + A high performance LRU implementation to overcommit memory.
    > +
    > source "mm/damon/Kconfig"

    This is a problem. I had to jump through hoops just to be able to
    compile-test this. Turns out I had to figure out how to disable
    MAXSMP.

    Can we please figure out a way to ensure that more testers are at least
    compile testing this? Allnoconfig, defconfig, allyesconfig, allmodconfig.

    Also, I suggest that we actually make MGLRU the default while in linux-next.

    >
    > ...
    >

    \
     
     \ /
      Last update: 2022-04-12 04:17    [W:3.561 / U:0.220 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site