linux-kernel - Re: [PATCH] mm/zswap: Check all pool pages instead of one pool pages

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Message-ID: <20140114045022.GZ1992@bbox>
Date:	Tue, 14 Jan 2014 13:50:22 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	Cai Liu <cai.liu@...sung.com>, sjenning@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, liucai.lfn@...il.com
Subject: Re: [PATCH] mm/zswap: Check all pool pages instead of one pool pages

Hello Bob,

On Tue, Jan 14, 2014 at 09:19:23AM +0800, Bob Liu wrote:
> 
> On 01/14/2014 07:35 AM, Minchan Kim wrote:
> > Hello,
> > 
> > On Sat, Jan 11, 2014 at 03:43:07PM +0800, Cai Liu wrote:
> >> zswap can support multiple swapfiles. So we need to check
> >> all zbud pool pages in zswap.
> > 
> > True but this patch is rather costly that we should iterate
> > zswap_tree[MAX_SWAPFILES] to check it. SIGH.
> > 
> > How about defining zswap_tress as linked list instead of static
> > array? Then, we could reduce unnecessary iteration too much.
> > 
> 
> But if use linked list, it might not easy to access the tree like this:
> struct zswap_tree *tree = zswap_trees[type];

struct zswap_tree {
    ..
    ..
    struct list_head list;
}

zswap_frontswap_init()
{
    ..
    ..
    zswap_trees[type] = tree;
    list_add(&tree->list, &zswap_list);
}

get_zswap_pool_pages(void)
{
    struct zswap_tree *cur;
    list_for_each_entry(cur, &zswap_list, list) {
        pool_pages += zbud_get_pool_size(cur->pool);
    }
    return pool_pages;
}


> 
> BTW: I'm still prefer to use dynamic pool size, instead of use
> zswap_is_full(). AFAIR, Seth has a plan to replace the rbtree with radix
> which will be more flexible to support this feature and page migration
> as well.
> 
> > Other question:
> > Why do we need to update zswap_pool_pages too frequently?
> > As I read the code, I think it's okay to update it only when user
> > want to see it by debugfs and zswap_is_full is called.
> > So could we optimize it out?
> > 
> >>
> >> Signed-off-by: Cai Liu <cai.liu@...sung.com>
> 
> Reviewed-by: Bob Liu <bob.liu@...cle.com>

Hmm, I really suprised you are okay in this code piece where we have
unnecessary cost most of case(ie, most system has a swap device) in
*mm* part.

Anyway, I don't want to merge this patchset.
If Andrew merge it and anybody doesn't do right work, I will send a patch.
Cai, Could you redo a patch?
I don't want to intercept your credit.

Even, we could optimize to reduce the the number of call as I said in
previous reply.

Thanks.

> 
> >> ---
> >>  mm/zswap.c |   18 +++++++++++++++---
> >>  1 file changed, 15 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index d93afa6..2438344 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -291,7 +291,6 @@ 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);
> >>  }
> >>  
> >>  /* caller must hold the tree lock */
> >> @@ -405,10 +404,24 @@ cleanup:
> >>  /*********************************
> >>  * helpers
> >>  **********************************/
> >> +static u64 get_zswap_pool_pages(void)
> >> +{
> >> +	int i;
> >> +	u64 pool_pages = 0;
> >> +
> >> +	for (i = 0; i < MAX_SWAPFILES; i++) {
> >> +		if (zswap_trees[i])
> >> +			pool_pages += zbud_get_pool_size(zswap_trees[i]->pool);
> >> +	}
> >> +	zswap_pool_pages = pool_pages;
> >> +
> >> +	return pool_pages;
> >> +}
> >> +
> >>  static bool zswap_is_full(void)
> >>  {
> >>  	return (totalram_pages * zswap_max_pool_percent / 100 <
> >> -		zswap_pool_pages);
> >> +		get_zswap_pool_pages());
> >>  }
> >>  
> >>  /*********************************
> >> @@ -716,7 +729,6 @@ 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);
> >>  
> >>  	return 0;
> >>  
> >> -- 
> >> 1.7.10.4
> -- 
> Regards,
> -Bob
> 
> --
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-- 
Kind regards,
Minchan Kim
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