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