Re: [PATCH v2] mm/zswap: Check all pool pages instead of one poolpages

From: Minchan Kim
Date: Tue Jan 21 2014 - 03:17:27 EST

Hello,

On Tue, Jan 21, 2014 at 02:35:07PM +0800, Cai Liu wrote:
> 2014/1/21 Minchan Kim <minchan@xxxxxxxxxx>:
> > Please check your MUA and don't break thread.
> >
> > On Tue, Jan 21, 2014 at 11:07:42AM +0800, Cai Liu wrote:
> >> Thanks for your review.
> >>
> >> 2014/1/21 Minchan Kim <minchan@xxxxxxxxxx>:
> >> > Hello Cai,
> >> >
> >> > On Mon, Jan 20, 2014 at 03:50:18PM +0800, Cai Liu wrote:
> >> >> zswap can support multiple swapfiles. So we need to check
> >> >> all zbud pool pages in zswap.
> >> >>
> >> >> Version 2:
> >> >> * add *total_zbud_pages* in zbud to record all the pages in pools
> >> >> * move the updating of pool pages statistics to
> >> >> alloc_zbud_page/free_zbud_page to hide the details
> >> >>
> >> >> Signed-off-by: Cai Liu <cai.liu@xxxxxxxxxxx>
> >> >> ---
> >> >> include/linux/zbud.h | 2 +-
> >> >> mm/zbud.c | 44 ++++++++++++++++++++++++++++++++------------
> >> >> mm/zswap.c | 4 ++--
> >> >> 3 files changed, 35 insertions(+), 15 deletions(-)
> >> >>
> >> >> diff --git a/include/linux/zbud.h b/include/linux/zbud.h
> >> >> index 2571a5c..1dbc13e 100644
> >> >> --- a/include/linux/zbud.h
> >> >> +++ b/include/linux/zbud.h
> >> >> @@ -17,6 +17,6 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle);
> >> >> int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries);
> >> >> void *zbud_map(struct zbud_pool *pool, unsigned long handle);
> >> >> void zbud_unmap(struct zbud_pool *pool, unsigned long handle);
> >> >> -u64 zbud_get_pool_size(struct zbud_pool *pool);
> >> >> +u64 zbud_get_pool_size(void);
> >> >>
> >> >> #endif /* _ZBUD_H_ */
> >> >> diff --git a/mm/zbud.c b/mm/zbud.c
> >> >> index 9451361..711aaf4 100644
> >> >> --- a/mm/zbud.c
> >> >> +++ b/mm/zbud.c
> >> >> @@ -52,6 +52,13 @@
> >> >> #include <linux/spinlock.h>
> >> >> #include <linux/zbud.h>
> >> >>
> >> >> +/*********************************
> >> >> +* statistics
> >> >> +**********************************/
> >> >> +
> >> >> +/* zbud pages in all pools */
> >> >> +static u64 total_zbud_pages;
> >> >> +
> >> >> /*****************
> >> >> * Structures
> >> >> *****************/
> >> >> @@ -142,10 +149,28 @@ static struct zbud_header *init_zbud_page(struct page *page)
> >> >> return zhdr;
> >> >> }
> >> >>
> >> >> +static struct page *alloc_zbud_page(struct zbud_pool *pool, gfp_t gfp)
> >> >> +{
> >> >> + struct page *page;
> >> >> +
> >> >> + page = alloc_page(gfp);
> >> >> +
> >> >> + if (page) {
> >> >> + pool->pages_nr++;
> >> >> + total_zbud_pages++;
> >> >
> >> > Who protect race?
> >>
> >> Yes, here the pool->pages_nr and also the total_zbud_pages are not protected.
> >> I will re-do it.
> >>
> >> I will change *total_zbud_pages* to atomic type.
> >
> > Wait, it doesn't make sense. Now, you assume zbud allocator would be used
> > for only zswap. It's true until now but we couldn't make sure it in future.
> > If other user start to use zbud allocator, total_zbud_pages would be pointless.
>
> Yes, you are right. ZBUD is a common module. So in this patch calculate the
> zswap pool size in zbud is not suitable.
>
> >
> > Another concern is that what's your scenario for above two swap?
> > How often we need to call zbud_get_pool_size?
> > In previous your patch, you reduced the number of call so IIRC,
> > we only called it in zswap_is_full and for debugfs.
>
> zbud_get_pool_size() is called frequently when adding/freeing zswap
> entry happen in zswap . This is why in this patch I added a counter in zbud,
> and then in zswap the iteration of zswap_list to calculate the pool size will
> not be needed.

We can remove updating zswap_pool_pages in zswap_frontswap_store and
zswap_free_entry as I said. So zswap_is_full is only hot spot.
Do you think it's still big overhead? Why? Maybe locking to prevent
destroying? Then, we can use RCU to minimize the overhead as I mentioned.

>
> > Of course, it would need some lock or refcount to prevent destroy
> > of zswap_tree in parallel with zswap_frontswap_invalidate_area but
> > zswap_is_full doesn't need to be exact so RCU would be good fit.
> >
> > Most important point is that now zswap doesn't consider multiple swap.
> > For example, Let's assume you uses two swap A and B with different priority
> > and A already has charged 19% long time ago and let's assume that A swap is
> > full now so VM start to use B so that B has charged 1% recently.
> > It menas zswap charged (19% + 1%)i is full by default.
> >
> > Then, if VM want to swap out more pages into B, zbud_reclaim_page
> > would be evict one of pages in B's pool and it would be repeated
> > continuously. It's totally LRU reverse problem and swap thrashing in B
> > would happen.
> >
>
> The scenario is below:
> There are 2 swap A, B in system. If pool size of A reach 19% of ram
> size and swap A
> is also full. Then swap B will be used. Pool size of B will be
> increased until it hit
> the 20% of the ram size. By now zswap pool size is about 39% of ram size.
> If there are more than 2 swap file/device, zswap pool will expand out
> of control
> and there may be no swapout happened.

I know.

>
> I think the original intention of zswap designer is to keep the total
> zswap pools size below
> 20% of RAM size.

My point is your patch still doesn't solve the example I mentioned.

>
> Thanks.
>
> > Please say your usecase scenario and if it's really problem,
> > we need more surgery.
> >
> > Thanks.
> >
> >> For *pool->pages_nr*, one way is to use pool->lock to protect. But I
> >> think it is too heavy.
> >> So does it ok to change pages_nr to atomic type too?
> >>
> >>
> >> >
> >> >> + }
> >> >> +
> >> >> + return page;
> >> >> +}
> >> >> +
> >> >> +
> >> >> /* Resets the struct page fields and frees the page */
> >> >> -static void free_zbud_page(struct zbud_header *zhdr)
> >> >> +static void free_zbud_page(struct zbud_pool *pool, struct zbud_header *zhdr)
> >> >> {
> >> >> __free_page(virt_to_page(zhdr));
> >> >> +
> >> >> + pool->pages_nr--;
> >> >> + total_zbud_pages--;
> >> >> }
> >> >>
> >> >> /*
> >> >> @@ -279,11 +304,10 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> >> >>
> >> >> /* Couldn't find unbuddied zbud page, create new one */
> >> >> spin_unlock(&pool->lock);
> >> >> - page = alloc_page(gfp);
> >> >> + page = alloc_zbud_page(pool, gfp);
> >> >> if (!page)
> >> >> return -ENOMEM;
> >> >> spin_lock(&pool->lock);
> >> >> - pool->pages_nr++;
> >> >> zhdr = init_zbud_page(page);
> >> >> bud = FIRST;
> >> >>
> >> >> @@ -349,8 +373,7 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> >> >> if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> >> >> /* zbud page is empty, free */
> >> >> list_del(&zhdr->lru);
> >> >> - free_zbud_page(zhdr);
> >> >> - pool->pages_nr--;
> >> >> + free_zbud_page(pool, zhdr);
> >> >> } else {
> >> >> /* Add to unbuddied list */
> >> >> freechunks = num_free_chunks(zhdr);
> >> >> @@ -447,8 +470,7 @@ next:
> >> >> * Both buddies are now free, free the zbud page and
> >> >> * return success.
> >> >> */
> >> >> - free_zbud_page(zhdr);
> >> >> - pool->pages_nr--;
> >> >> + free_zbud_page(pool, zhdr);
> >> >> spin_unlock(&pool->lock);
> >> >> return 0;
> >> >> } else if (zhdr->first_chunks == 0 ||
> >> >> @@ -496,14 +518,12 @@ void zbud_unmap(struct zbud_pool *pool, unsigned long handle)
> >> >>
> >> >> /**
> >> >> * zbud_get_pool_size() - gets the zbud pool size in pages
> >> >> - * @pool: pool whose size is being queried
> >> >> *
> >> >> - * Returns: size in pages of the given pool. The pool lock need not be
> >> >> - * taken to access pages_nr.
> >> >> + * Returns: size in pages of all the zbud pools.
> >> >> */
> >> >> -u64 zbud_get_pool_size(struct zbud_pool *pool)
> >> >> +u64 zbud_get_pool_size(void)
> >> >> {
> >> >> - return pool->pages_nr;
> >> >> + return total_zbud_pages;
> >> >> }
> >> >>
> >> >> static int __init init_zbud(void)
> >> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> >> index 5a63f78..ef44d9d 100644
> >> >> --- a/mm/zswap.c
> >> >> +++ b/mm/zswap.c
> >> >> @@ -291,7 +291,7 @@ static void zswap_free_entry(struct zswap_tree *tree,
> >> >> zbud_free(tree->pool, entry->handle);
> >> >> zswap_entry_cache_free(entry);
> >> >> atomic_dec(&zswap_stored_pages);
> >> >> - zswap_pool_pages = zbud_get_pool_size(tree->pool);
> >> >> + zswap_pool_pages = zbud_get_pool_size();
> >> >> }
> >> >>
> >> >> /* caller must hold the tree lock */
> >> >> @@ -716,7 +716,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> >> >>
> >> >> /* update stats */
> >> >> atomic_inc(&zswap_stored_pages);
> >> >> - zswap_pool_pages = zbud_get_pool_size(tree->pool);
> >> >> + zswap_pool_pages = zbud_get_pool_size();
> >> >>
> >> >> return 0;
> >> >>
> >> >> --
> >> >> 1.7.10.4
> >> >>
> >> >> --
> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> >> see: http://www.linux-mm.org/ .
> >> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >> >
> >> > --
> >> > Kind regards,
> >> > Minchan Kim
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

--
Kind regards,
Minchan Kim
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