linux-kernel - Re: [PATCH 2/5] writeback: charge leaked page dirties to active tasks

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Date:	Mon, 21 Nov 2011 13:49:29 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	<linux-fsdevel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] writeback: charge leaked page dirties to active
 tasks

On Mon, 21 Nov 2011 21:03:44 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> It's a years long problem that a large number of short-lived dirtiers
> (eg. gcc instances in a fast kernel build) may starve long-run dirtiers
> (eg. dd) as well as pushing the dirty pages to the global hard limit.
> 
> The solution is to charge the pages dirtied by the exited gcc to the
> other random dirtying tasks. It sounds not perfect, however should
> behave good enough in practice, seeing as that throttled tasks aren't
> actually running so those that are running are more likely to pick it up
> and get throttled, therefore promoting an equal spread.
> 
> --- linux-next.orig/mm/page-writeback.c	2011-11-17 20:57:04.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-11-17 20:57:13.000000000 +0800
> @@ -1194,6 +1194,7 @@ void set_page_dirty_balance(struct page 
>  }
>  
>  static DEFINE_PER_CPU(int, bdp_ratelimits);
> +DEFINE_PER_CPU(int, dirty_leaks) = 0;

This is a poor identifier for a global symbol.  Generally such symbols
should at least identify what subsystem they belong to.

Also, this would be a good site at whcih to document the global
symbol's role.  The writeback code needs a lot of documentation. Of
the design-level kind.

>  /**
>   * balance_dirty_pages_ratelimited_nr - balance dirty memory state
> @@ -1242,6 +1243,17 @@ void balance_dirty_pages_ratelimited_nr(
>  			ratelimit = 0;
>  		}
>  	}
> +	/*
> +	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
> +	 * short-lived tasks (eg. gcc invocations in a kernel build) escaping
> +	 * the dirty throttling and livelock other long-run dirtiers.
> +	 */
> +	p = &__get_cpu_var(dirty_leaks);
> +	if (*p > 0 && current->nr_dirtied < ratelimit) {
> +		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
> +		*p -= nr_pages_dirtied;
> +		current->nr_dirtied += nr_pages_dirtied;
> +	}
>  	preempt_enable();
>  
>  	if (unlikely(current->nr_dirtied >= ratelimit))
> --- linux-next.orig/kernel/exit.c	2011-11-17 20:57:02.000000000 +0800
> +++ linux-next/kernel/exit.c	2011-11-17 20:57:04.000000000 +0800
> @@ -1037,6 +1037,8 @@ NORET_TYPE void do_exit(long code)
>  	validate_creds_for_do_exit(tsk);
>  
>  	preempt_disable();
> +	if (tsk->nr_dirtied)
> +		__this_cpu_add(dirty_leaks, tsk->nr_dirtied);

Whatever problem this code is solving, it only solved it in certain
cases.  For example, if tasks are forking, dirtying and exiting at a
rapid rate on CPU 0 then all the other CPUs don't know anything about
this and we didn't fix anything.

IOW, it all seems like a half-assed bandaid.
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