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SubjectRe: [PATCH 15/18] writeback: charge leaked page dirties to active tasks
On Sun 04-09-11 09:53:20, 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.
I don't think it's years long problem. When we do per-cpu ratelimiting,
short lived processes have the same chance (proportional to the number of
pages dirtied) of hitting balance_dirty_pages() as long-run dirtiers have.
So this problem seems to be introduced by your per task dirty ratelimiting?
But given that you kept per-cpu ratelimiting in the end, is this still an
issue? Do you have some numbers for this patch?

Honza

> The solution is to charge the pages dirtied by the exited gcc to the
> other random gcc/dd instances. It sounds not perfect, however should
> behave good enough in practice.
>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> include/linux/writeback.h | 2 ++
> kernel/exit.c | 2 ++
> mm/page-writeback.c | 12 ++++++++++++
> 3 files changed, 16 insertions(+)
>
> --- linux-next.orig/include/linux/writeback.h 2011-08-29 19:14:22.000000000 +0800
> +++ linux-next/include/linux/writeback.h 2011-08-29 19:14:32.000000000 +0800
> @@ -7,6 +7,8 @@
> #include <linux/sched.h>
> #include <linux/fs.h>
>
> +DECLARE_PER_CPU(int, dirty_leaks);
> +
> /*
> * The 1/4 region under the global dirty thresh is for smooth dirty throttling:
> *
> --- linux-next.orig/mm/page-writeback.c 2011-08-29 19:14:22.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-08-29 19:14:32.000000000 +0800
> @@ -1237,6 +1237,7 @@ void set_page_dirty_balance(struct page
> }
>
> static DEFINE_PER_CPU(int, bdp_ratelimits);
> +DEFINE_PER_CPU(int, dirty_leaks) = 0;
>
> /**
> * balance_dirty_pages_ratelimited_nr - balance dirty memory state
> @@ -1285,6 +1286,17 @@ 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 2011-08-26 16:19:27.000000000 +0800
> +++ linux-next/kernel/exit.c 2011-08-29 19:14:22.000000000 +0800
> @@ -1044,6 +1044,8 @@ 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);
> exit_rcu();
> /* causes final put_task_struct in finish_task_switch(). */
> tsk->state = TASK_DEAD;
>
>
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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