Re: [PATCH 4/5] writeback: per task dirty rate limit

Andrea Righi
2011-08-13T16:28:40Z

On Mon, Aug 08, 2011 at 10:23:18PM +0800, Wu Fengguang wrote:
> On Mon, Aug 08, 2011 at 09:47:14PM +0800, Peter Zijlstra wrote:
> > On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > > Add two fields to task_struct.
> > >
> > > 1) account dirtied pages in the individual tasks, for accuracy
> > > 2) per-task balance_dirty_pages() call intervals, for flexibility
> > >
> > > The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
> > > scale near-sqrt to the safety gap between dirty pages and threshold.
> > >
> > > XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
> > > dirtying pages at exactly the same time, each task will be assigned a
> > > large initial nr_dirtied_pause, so that the dirty threshold will be
> > > exceeded long before each task reached its nr_dirtied_pause and hence
> > > call balance_dirty_pages().
> > >
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > > include/linux/sched.h | 7 ++
> > > mm/memory_hotplug.c | 3 -
> > > mm/page-writeback.c | 106 +++++++++-------------------------------
> > > 3 files changed, 32 insertions(+), 84 deletions(-)
> >
> > No fork() hooks? This way tasks inherit their parent's dirty count on
> > clone().
>
> btw, I do have another patch queued for improving the "leaked dirties
> on exit" case :)
>
> Thanks,
> Fengguang
> ---
> Subject: writeback: charge leaked page dirties to active tasks
> Date: Tue Apr 05 13:21:19 CST 2011
>
> 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 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 | 11 +++++++++++
> 3 files changed, 15 insertions(+)
>
> --- linux-next.orig/include/linux/writeback.h 2011-08-08 21:45:58.000000000 +0800
> +++ linux-next/include/linux/writeback.h 2011-08-08 21:45:58.000000000 +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 2011-08-08 21:45:58.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-08-08 22:21:50.000000000 +0800
> @@ -190,6 +190,7 @@ int dirty_ratio_handler(struct ctl_table
> return ret;
> }
>
> +DEFINE_PER_CPU(int, dirty_leaks) = 0;
>
> int dirty_bytes_handler(struct ctl_table *table, int write,
> void __user *buffer, size_t *lenp,
> @@ -1150,6 +1151,7 @@ void balance_dirty_pages_ratelimited_nr(
> {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> int ratelimit;
> + int *p;
>
> if (!bdi_cap_account_dirty(bdi))
> return;
> @@ -1158,6 +1160,15 @@ void balance_dirty_pages_ratelimited_nr(
> if (bdi->dirty_exceeded)
> ratelimit = 8;
>
> + preempt_disable();
> + 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();
> +

I think we are still leaking some dirty pages, when the condition is
false nr_pages_dirtied is just ignored.

Why not doing something like this?

current->nr_dirtied += nr_pages_dirtied;
if (current->nr_dirtied < ratelimit) {
p = &get_cpu_var(dirty_leaks);
if (*p > 0) {
nr_pages_dirtied = min(*p, ratelimit -
current->nr_dirtied);
*p -= nr_pages_dirtied;
} else
nr_pages_dirtied = 0;
put_cpu_var(dirty_leaks);

current->nr_dirtied += nr_pages_dirtied;
}

Thanks,
-Andrea

> if (unlikely(current->nr_dirtied >= ratelimit))
> balance_dirty_pages(mapping, current->nr_dirtied);
> }
> --- linux-next.orig/kernel/exit.c 2011-08-08 21:43:37.000000000 +0800
> +++ linux-next/kernel/exit.c 2011-08-08 21:45:58.000000000 +0800
> @@ -1039,6 +1039,8 @@ 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;