Re: [RFC 1/3] zsmalloc: move pages_allocated to zs_pool
From: Sergey Senozhatsky <hidden>
Date: 2014-08-14 13:04:56
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On (08/13/14 12:11), Dan Streetman wrote:
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On Wed, Aug 13, 2014 at 10:14 AM, Sergey Senozhatsky [off-list ref] wrote:quoted
On (08/13/14 09:59), Dan Streetman wrote:quoted
On Tue, Aug 5, 2014 at 4:02 AM, Minchan Kim [off-list ref] wrote:quoted
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'm missing where that is? I don't see that in this patch?quoted
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.Do you mean this from the 2/3 patch:
yeah. sorry for being unclear, I was really sleepy.
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@@ -946,6 +947,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size) set_zspage_mapping(first_page, class->index, ZS_EMPTY); spin_lock(&pool->stat_lock); pool->pages_allocated += class->pages_per_zspage; + if (pool->max_pages_allocated < pool->pages_allocated) + pool->max_pages_allocated = pool->pages_allocated; spin_unlock(&pool->stat_lock); spin_lock(&class->lock); }I see, yeah the max > allocated check before setting is easiest done with a spinlock. I think pages_allocated could still be done as atomic, just using atomic_add_return() to grab the current value to check against, but keeping them the same type and both protected by the same spinlock I guess simplifies things. Although, if they were both atomic, then the *only* place that would need a spinlock would be this check - reading the (atomic) max_pages_allocated wouldn't need a spinlock, nor would clearing it to 0.
makes sense. -ss
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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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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