Date:   Mon, 24 Apr 2017 14:47:11 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

Minchan Kim <minchan@...nel.org> writes:

> On Fri, Apr 21, 2017 at 08:29:30PM +0800, Huang, Ying wrote:
>> "Huang, Ying" <ying.huang@...el.com> writes:
>> 
>> > Minchan Kim <minchan@...nel.org> writes:
>> >
>> >> On Wed, Apr 19, 2017 at 04:14:43PM +0800, Huang, Ying wrote:
>> >>> Minchan Kim <minchan@...nel.org> writes:
>> >>> 
>> >>> > Hi Huang,
>> >>> >
>> >>> > On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote:
>> >>> >> From: Huang Ying <ying.huang@...el.com>
>> >>> >> 
>> >>> >>  void swapcache_free_entries(swp_entry_t *entries, int n)
>> >>> >>  {
>> >>> >>  	struct swap_info_struct *p, *prev;
>> >>> >> @@ -1075,6 +1083,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
>> >>> >>  
>> >>> >>  	prev = NULL;
>> >>> >>  	p = NULL;
>> >>> >> +
>> >>> >> +	/* Sort swap entries by swap device, so each lock is only taken once. */
>> >>> >> +	if (nr_swapfiles > 1)
>> >>> >> +		sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);
>> >>> >
>> >>> > Let's think on other cases.
>> >>> >
>> >>> > There are two swaps and they are configured by priority so a swap's usage
>> >>> > would be zero unless other swap used up. In case of that, this sorting
>> >>> > is pointless.
>> >>> >
>> >>> > As well, nr_swapfiles is never decreased so if we enable multiple
>> >>> > swaps and then disable until a swap is remained, this sorting is
>> >>> > pointelss, too.
>> >>> >
>> >>> > How about lazy sorting approach? IOW, if we found prev != p and,
>> >>> > then we can sort it.
>> >>> 
>> >>> Yes.  That should be better.  I just don't know whether the added
>> >>> complexity is necessary, given the array is short and sort is fast.
>> >>
>> >> Huh?
>> >>
>> >> 1. swapon /dev/XXX1
>> >> 2. swapon /dev/XXX2
>> >> 3. swapoff /dev/XXX2
>> >> 4. use only one swap
>> >> 5. then, always pointless sort.
>> >
>> > Yes.  In this situation we will do unnecessary sorting.  What I don't
>> > know is whether the unnecessary sorting will hurt performance in real
>> > life.  I can do some measurement.
>> 
>> I tested the patch with 1 swap device and 1 process to eat memory
>> (remove the "if (nr_swapfiles > 1)" for test).  I think this is the
>> worse case because there is no lock contention.  The memory freeing time
>> increased from 1.94s to 2.12s (increase ~9.2%).  So there is some
>> overhead for some cases.  I change the algorithm to something like
>> below,
>> 
>>  void swapcache_free_entries(swp_entry_t *entries, int n)
>>  {
>>  	struct swap_info_struct *p, *prev;
>>  	int i;
>> +	swp_entry_t entry;
>> +	unsigned int prev_swp_type;
>>  
>>  	if (n <= 0)
>>  		return;
>>  
>> +	prev_swp_type = swp_type(entries[0]);
>> +	for (i = n - 1; i > 0; i--) {
>> +		if (swp_type(entries[i]) != prev_swp_type)
>> +			break;
>> +	}
>
> That's really what I want to avoid. For many swap usecases,
> it adds unnecessary overhead.
>
>> +
>> +	/* Sort swap entries by swap device, so each lock is only taken once. */
>> +	if (i)
>> +		sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);
>>  	prev = NULL;
>>  	p = NULL;
>>  	for (i = 0; i < n; ++i) {
>> -		p = swap_info_get_cont(entries[i], prev);
>> +		entry = entries[i];
>> +		p = swap_info_get_cont(entry, prev);
>>  		if (p)
>> -			swap_entry_free(p, entries[i]);
>> +			swap_entry_free(p, entry);
>>  		prev = p;
>>  	}
>>  	if (p)
>> 
>> With this patch, the memory freeing time increased from 1.94s to 1.97s.
>> I think this is good enough.  Do you think so?
>
> What I mean is as follows(I didn't test it at all):
>
> With this, sort entries if we found multiple entries in current
> entries. It adds some condition checks for non-multiple swap
> usecase but it would be more cheaper than the sorting.
> And it adds a [un]lock overhead for multiple swap usecase but
> it should be a compromise for single-swap usecase which is more
> popular.

Yes.  What I concerned is that one swap device may be locked twice
instead of once during the freeing.  I will give it some test.

Best Regards,
Huang, Ying

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f23c56e9be39..0d76a492786f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1073,30 +1073,40 @@ static int swp_entry_cmp(const void *ent1, const void *ent2)
>  	return (long)(swp_type(*e1) - swp_type(*e2));
>  }
>  
> -void swapcache_free_entries(swp_entry_t *entries, int n)
> +void swapcache_free_entries(swp_entry_t *entries, int nr)
>  {
> -	struct swap_info_struct *p, *prev;
>  	int i;
> +	struct swap_info_struct *cur, *prev = NULL;
> +	bool sorted = false;
>  
> -	if (n <= 0)
> +	if (nr <= 0)
>  		return;
>  
> -	prev = NULL;
> -	p = NULL;
> -
> -	/* Sort swap entries by swap device, so each lock is only taken once. */
> -	if (nr_swapfiles > 1)
> -		sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);
> -	for (i = 0; i < n; ++i) {
> -		p = swap_info_get_cont(entries[i], prev);
> -		if (p)
> -			swap_entry_free(p, entries[i]);
> -		else
> +	for (i = 0; i < nr; i++) {
> +		cur = swap_info_get_cont(entries[i], prev);
> +		if (!cur)
>  			break;
> -		prev = p;
> +		if (cur != prev && !sorted && prev) {
> +			spin_unlock(&cur->lock);
> +			/*
> +			 * Sort swap entries by swap device,
> +			 * so each lock is only taken once.
> +			 */
> +			sort(entries + i, nr - i,
> +					sizeof(swp_entry_t),
> +					swp_entry_cmp, NULL);
> +			sorted = true;
> +			prev = NULL;
> +			i--;
> +			continue;
> +		}
> +
> +		swap_entry_free(cur, entries[i]);
> +		prev = cur;
>  	}
> -	if (p)
> -		spin_unlock(&p->lock);
> +
> +	if (cur)
> +		spin_unlock(&cur->lock);
>  }
>  
>  /*

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