Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932125AbaAUNwa (ORCPT ); Tue, 21 Jan 2014 08:52:30 -0500 Received: from mail-qa0-f44.google.com ([209.85.216.44]:53280 "EHLO mail-qa0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932076AbaAUNw1 (ORCPT ); Tue, 21 Jan 2014 08:52:27 -0500 MIME-Version: 1.0 In-Reply-To: <20140121081820.GA31230@bbox> References: <20140121050439.GA16664@bbox> <20140121081820.GA31230@bbox> Date: Tue, 21 Jan 2014 21:52:25 +0800 Message-ID: Subject: Re: [PATCH v2] mm/zswap: Check all pool pages instead of one pool pages From: Cai Liu To: Minchan Kim Cc: Seth Jennings , Bob Liu , Cai Liu , Andrew Morton , Linux-MM , Linux-Kernel Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Minchan 2014/1/21 Minchan Kim : > Hello, > > On Tue, Jan 21, 2014 at 02:35:07PM +0800, Cai Liu wrote: >> 2014/1/21 Minchan Kim : >> > 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 : >> >> > 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 >> >> >> --- >> >> >> 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 >> >> >> #include >> >> >> >> >> >> +/********************************* >> >> >> +* 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. I get your point. Yes, In my previous patch, zswap_is_full() was the only path to call zbud_get_pool_size(). And your suggestion on patch v1 to remove the unnecessary iteration will reduce the overhead further. So adding the calculating of all the pool size in zswap.c is better. >> >> > 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. Hmm. My patch only make sure all the zswap pools use maximum 20% of RAM size. It is a new problem in your example. The zbud_reclaim_page would not swap out the oldest zbud page when multiple swaps are used. Maybe the new problem can be resolved in another patch. Thanks. > >> >> 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@kvack.org. For more info on Linux MM, >> >> >> see: http://www.linux-mm.org/ . >> >> >> Don't email: email@kvack.org >> >> > >> >> > -- >> >> > Kind regards, >> >> > Minchan Kim >> >> >> >> -- >> >> 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: email@kvack.org >> > >> > -- >> > Kind regards, >> > Minchan Kim >> >> -- >> 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: email@kvack.org > > -- > Kind regards, > Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/