Re: [PATCH] zsmalloc: allow only one active pool compaction context

[Yosry Ahmed <yosryahmed@google.com>]

Hi Sergey,

On Mon, Apr 17, 2023 at 4:03 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> zsmalloc pool can be compacted concurrently by many contexts,
> e.g.
>
>  cc1 handle_mm_fault()
>       do_anonymous_page()
>        __alloc_pages_slowpath()
>         try_to_free_pages()
>          do_try_to_free_pages(
>           lru_gen_shrink_node()
>            shrink_slab()
>             do_shrink_slab()
>              zs_shrinker_scan()
>               zs_compact()
>
> This creates unnecessary contention as all those processes
> compete for access to the same classes. A single compaction
> process is enough. Moreover contention that is created by
> multiple compaction processes impact other zsmalloc functions,
> e.g. zs_malloc(), since zsmalloc uses "global" pool->lock to
> synchronize access to pool.
>
> Introduce pool compaction mutex and permit only one compaction
> context at a time.

I am not sure what's the best practice here, but if the only use of
the mutex is a trylock, do we need a mutex here? It seems like a
simple atomic would do the trick. Perhaps something like:

static atomic_t ongoing_compaction = ATOMIC_INIT(0);
...
if (atomic_xchg(&ongoing_compaction, 1))
    return;
....
atomic_set(&ongoing_compaction, 0);

FWIW, I am suggesting this because it was recently brought up when one
of my patches was reviewed that locks should protect data not code
paths [1], so I am trying to apply what I learned there :)

[1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/

>
> /proc/lock-stat after make -j$((`nproc`+1)) linux kernel for
> &pool->lock#3:
>
>                 Base           Patched
> --------------------------------------
> con-bounces     9797655        8125860
> contentions     11131185       9242153
> waittime-min    0.09           0.10
> waittime-max    4171695.76     3926258.74
> waittime-total  506197629.16   417061026.20
> waittime-avg    45.48          45.13
> acq-bounces     13809103       11383480
> acquisitions    21145155       18049364
> holdtime-min    0.06           0.07
> holdtime-max    7379928.80     3926274.89
> holdtime-total  46273950.89    37279624.53
> holdtime-avg    2.19           2.07
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  mm/zsmalloc.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index cc81dfba05a0..0e036ec56c3c 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -264,6 +264,7 @@ struct zs_pool {
>         struct work_struct free_work;
>  #endif
>         spinlock_t lock;
> +       struct mutex compact_lock;
>  };
>
>  struct zspage {
> @@ -2274,6 +2275,9 @@ unsigned long zs_compact(struct zs_pool *pool)
>         struct size_class *class;
>         unsigned long pages_freed = 0;
>
> +       if (!mutex_trylock(&pool->compact_lock))
> +               return 0;
> +
>         for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
>                 class = pool->size_class[i];
>                 if (class->index != i)
> @@ -2281,6 +2285,7 @@ unsigned long zs_compact(struct zs_pool *pool)
>                 pages_freed += __zs_compact(pool, class);
>         }
>         atomic_long_add(pages_freed, &pool->stats.pages_compacted);
> +       mutex_unlock(&pool->compact_lock);
>
>         return pages_freed;
>  }
> @@ -2388,6 +2393,7 @@ struct zs_pool *zs_create_pool(const char *name)
>
>         init_deferred_free(pool);
>         spin_lock_init(&pool->lock);
> +       mutex_init(&pool->compact_lock);
>
>         pool->name = kstrdup(name, GFP_KERNEL);
>         if (!pool->name)
> --
> 2.40.0.634.g4ca3ef3211-goog
>