Re: [RFC 1/3] zsmalloc: move pages_allocated to zs_pool

[Dan Streetman <ddstreet@ieee.org>]

On Wed, Aug 13, 2014 at 10:14 AM, Sergey Senozhatsky
<sergey.senozhatsky@gmail.com> wrote:
> On (08/13/14 09:59), Dan Streetman wrote:
>> On Tue, Aug 5, 2014 at 4:02 AM, Minchan Kim <minchan@kernel.org> wrote:
>> > Pages_allocated has counted in size_class structure and when user
>> > want to see total_size_bytes, it gathers all of value from each
>> > size_class to report the sum.
>> >
>> > It's not bad if user don't see the value often but if user start
>> > to see the value frequently, it would be not a good deal for
>> > performance POV.
>> >
>> > This patch moves the variable from size_class to zs_pool so it would
>> > reduce memory footprint (from [255 * 8byte] to [sizeof(atomic_t)])
>> > but it adds new locking overhead but it wouldn't be severe because
>> > it's not a hot path in zs_malloc(ie, it is called only when new
>> > zspage is created, not a object).
>>
>> Would using an atomic64_t without locking be simpler?
>
> it would be racy.

oh.  atomic operations aren't smp safe?  is that because other
processors might use a stale value, and barriers must be added?  I
guess I don't quite understand the value of atomic then. :-/

>
>         -ss
>
>>
>> >
>> > Signed-off-by: Minchan Kim <minchan@kernel.org>
>> > ---
>> >  mm/zsmalloc.c | 30 ++++++++++++++++--------------
>> >  1 file changed, 16 insertions(+), 14 deletions(-)
>> >
>> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> > index fe78189624cf..a6089bd26621 100644
>> > --- a/mm/zsmalloc.c
>> > +++ b/mm/zsmalloc.c
>> > @@ -198,9 +198,6 @@ struct size_class {
>> >
>> >         spinlock_t lock;
>> >
>> > -       /* stats */
>> > -       u64 pages_allocated;
>> > -
>> >         struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
>> >  };
>> >
>> > @@ -216,9 +213,12 @@ struct link_free {
>> >  };
>> >
>> >  struct zs_pool {
>> > +       spinlock_t stat_lock;
>> > +
>> >         struct size_class size_class[ZS_SIZE_CLASSES];
>> >
>> >         gfp_t flags;    /* allocation flags used when growing pool */
>> > +       unsigned long pages_allocated;
>> >  };
>> >
>> >  /*
>> > @@ -882,6 +882,7 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>> >
>> >         }
>> >
>> > +       spin_lock_init(&pool->stat_lock);
>> >         pool->flags = flags;
>> >
>> >         return pool;
>> > @@ -943,8 +944,10 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>> >                         return 0;
>> >
>> >                 set_zspage_mapping(first_page, class->index, ZS_EMPTY);
>> > +               spin_lock(&pool->stat_lock);
>> > +               pool->pages_allocated += class->pages_per_zspage;
>> > +               spin_unlock(&pool->stat_lock);
>> >                 spin_lock(&class->lock);
>> > -               class->pages_allocated += class->pages_per_zspage;
>> >         }
>> >
>> >         obj = (unsigned long)first_page->freelist;
>> > @@ -997,14 +1000,14 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
>> >
>> >         first_page->inuse--;
>> >         fullness = fix_fullness_group(pool, first_page);
>> > -
>> > -       if (fullness == ZS_EMPTY)
>> > -               class->pages_allocated -= class->pages_per_zspage;
>> > -
>> >         spin_unlock(&class->lock);
>> >
>> > -       if (fullness == ZS_EMPTY)
>> > +       if (fullness == ZS_EMPTY) {
>> > +               spin_lock(&pool->stat_lock);
>> > +               pool->pages_allocated -= class->pages_per_zspage;
>> > +               spin_unlock(&pool->stat_lock);
>> >                 free_zspage(first_page);
>> > +       }
>> >  }
>> >  EXPORT_SYMBOL_GPL(zs_free);
>> >
>> > @@ -1100,12 +1103,11 @@ EXPORT_SYMBOL_GPL(zs_unmap_object);
>> >
>> >  u64 zs_get_total_size_bytes(struct zs_pool *pool)
>> >  {
>> > -       int i;
>> > -       u64 npages = 0;
>> > -
>> > -       for (i = 0; i < ZS_SIZE_CLASSES; i++)
>> > -               npages += pool->size_class[i].pages_allocated;
>> > +       u64 npages;
>> >
>> > +       spin_lock(&pool->stat_lock);
>> > +       npages = pool->pages_allocated;
>> > +       spin_unlock(&pool->stat_lock);
>> >         return npages << PAGE_SHIFT;
>> >  }
>> >  EXPORT_SYMBOL_GPL(zs_get_total_size_bytes);
>> > --
>> > 2.0.0
>> >
>> > --
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