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Date: Thu, 14 Aug 2014 00:25:04 +0900
From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To: Dan Streetman <ddstreet@...e.org>
Cc: Minchan Kim <minchan@...nel.org>, Linux-MM <linux-mm@...ck.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Jerome Marchand <jmarchan@...hat.com>, juno.choi@....com,
seungho1.park@....com, Luigi Semenzato <semenzato@...gle.com>,
Nitin Gupta <ngupta@...are.org>
Subject: Re: [RFC 1/3] zsmalloc: move pages_allocated to zs_pool
On (08/14/14 00:13), Sergey Senozhatsky wrote:
> > On Wed, Aug 13, 2014 at 10:14 AM, Sergey Senozhatsky
> > <sergey.senozhatsky@...il.com> wrote:
> > > On (08/13/14 09:59), Dan Streetman wrote:
> > >> On Tue, Aug 5, 2014 at 4:02 AM, Minchan Kim <minchan@...nel.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. :-/
>
> pool not only set the value, it also read it and make some decisions
> based on that value:
>
> pages_allocated += X
> if (pages_allocated >= max_pages_allocated)
> return 0;
I mean, suppose this happens on two CPUs
max_pages_allocated is 10; current pages_allocated is 8. now you have 2 zs_malloc()
happenning on two CPUs. each of them will do `pages_allocated += 1'. the problem is
that both will see 10 at `if (pages_allocated >= max_pages_allocated)', so we will
fail 2 operations, while we only were supposed to fail one.
-ss
>
> > >>
> > >> >
> > >> > Signed-off-by: Minchan Kim <minchan@...nel.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
> > >> >
> > >> > --
> > >> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > >> > the body to majordomo@...ck.org. For more info on Linux MM,
> > >> > see: http://www.linux-mm.org/ .
> > >> > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> > >>
> >
>
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