Re: [PATCH 2/7] writeback: charge leaked page dirties to active tasks

[Jan Kara <jack@suse.cz>]

On Mon 28-11-11 21:53:40, Wu Fengguang 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.
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
  Since I don't see a better solution here :)
Acked-by: Jan Kara <jack@suse.cz>

								Honza
> ---
>  include/linux/writeback.h |    2 ++
>  kernel/exit.c             |    2 ++
>  mm/page-writeback.c       |   27 +++++++++++++++++++++++++++
>  3 files changed, 31 insertions(+)
> 
> --- linux-next.orig/include/linux/writeback.h	2011-11-28 21:23:19.000000000 +0800
> +++ linux-next/include/linux/writeback.h	2011-11-28 21:23:20.000000000 +0800
> @@ -7,6 +7,8 @@
>  #include <linux/sched.h>
>  #include <linux/fs.h>
>  
> +DECLARE_PER_CPU(int, dirty_throttle_leaks);
> +
>  /*
>   * The 1/4 region under the global dirty thresh is for smooth dirty throttling:
>   *
> --- linux-next.orig/mm/page-writeback.c	2011-11-28 21:23:19.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-11-28 21:23:20.000000000 +0800
> @@ -1195,6 +1195,22 @@ void set_page_dirty_balance(struct page 
>  
>  static DEFINE_PER_CPU(int, bdp_ratelimits);
>  
> +/*
> + * Normal tasks are throttled by
> + *	loop {
> + *		dirty tsk->nr_dirtied_pause pages;
> + *		take a snap in balance_dirty_pages();
> + *	}
> + * However there is a worst case. If every task exit immediately when dirtied
> + * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
> + * called to throttle the page dirties. The solution is to save the not yet
> + * throttled page dirties in dirty_throttle_leaks on task exit and charge them
> + * randomly into the running tasks. This works well for the above worst case,
> + * as the new task will pick up and accumulate the old task's leaked dirty
> + * count and eventually get throttled.
> + */
> +DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
> +
>  /**
>   * balance_dirty_pages_ratelimited_nr - balance dirty memory state
>   * @mapping: address_space which was dirtied
> @@ -1242,6 +1258,17 @@ 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_throttle_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	2011-11-28 21:23:19.000000000 +0800
> +++ linux-next/kernel/exit.c	2011-11-28 21:23:20.000000000 +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_throttle_leaks, tsk->nr_dirtied);
>  	exit_rcu();
>  	/* causes final put_task_struct in finish_task_switch(). */
>  	tsk->state = TASK_DEAD;
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR