Date: Fri, 20 May 2011 12:30:04 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kamezawa.hiroyu@jp.fujitsu.com, riel@redhat.com
Subject: Re: [PATCH v2 3/3] vmscan: implement swap token priority aging
Message-Id: <20110520123004.e81c932e.akpm@linux-foundation.org>
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	<4DD481A7.3050108@jp.fujitsu.com>
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On Thu, 19 May 2011 11:34:15 +0900
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> While testing for memcg aware swap token, I observed a swap token
> was often grabbed an intermittent running process (eg init, auditd)
> and they never release a token.
> 
> Why?
> 
> Some processes (eg init, auditd, audispd) wake up when a process
> exiting. And swap token can be get first page-in process when
> a process exiting makes no swap token owner. Thus such above
> intermittent running process often get a token.
> 
> And currently, swap token priority is only decreased at page fault
> path. Then, if the process sleep immediately after to grab swap
> token, the swap token priority never be decreased. That's obviously
> undesirable.
> 
> This patch implement very poor (and lightweight) priority aging.
> It only be affect to the above corner case and doesn't change swap
> tendency workload performance (eg multi process qsbench load)
> 
> ...
>
> --- a/mm/thrash.c
> +++ b/mm/thrash.c
> @@ -25,10 +25,13 @@
> 
>  #include <trace/events/vmscan.h>
> 
> +#define TOKEN_AGING_INTERVAL	(0xFF)

Needs a comment describing its units and what it does, please. 
Sufficient for readers to understand why this value was chosen and what
effect they could expect to see from changing it.


>  static DEFINE_SPINLOCK(swap_token_lock);
>  struct mm_struct *swap_token_mm;
>  struct mem_cgroup *swap_token_memcg;
>  static unsigned int global_faults;
> +static unsigned int last_aging;

Is this a good name?  Would something like prev_global_faults be better?

`global_faults' and `last_aging' could be made static local in
grab_swap_token().

>  void grab_swap_token(struct mm_struct *mm)
>  {
> @@ -47,6 +50,11 @@ void grab_swap_token(struct mm_struct *mm)
>  	if (!swap_token_mm)
>  		goto replace_token;
> 
> +	if ((global_faults - last_aging) > TOKEN_AGING_INTERVAL) {
> +		swap_token_mm->token_priority /= 2;
> +		last_aging = global_faults;
> +	}

It's really hard to reverse-engineer the design decisions from the
implementation here, therefore...  ?

>  	if (mm == swap_token_mm) {
>  		mm->token_priority += 2;
>  		goto update_priority;
> @@ -64,7 +72,7 @@ void grab_swap_token(struct mm_struct *mm)
>  		goto replace_token;
> 
>  update_priority:
> -	trace_update_swap_token_priority(mm, old_prio);
> +	trace_update_swap_token_priority(mm, old_prio, swap_token_mm);
> 
>  out:
>  	mm->faultstamp = global_faults;
> @@ -80,6 +88,7 @@ replace_token:
>  	trace_replace_swap_token(swap_token_mm, mm);
>  	swap_token_mm = mm;
>  	swap_token_memcg = memcg;
> +	last_aging = global_faults;
>  	goto out;
>  }

In fact all of grab_swap_token() and the thrash-detection code in
general are pretty tricky and unobvious stuff.  So we left it
undocumented :(
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