On Wed, Feb 1, 2012 at 10:33 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> Hi Greg,
>
> On Wed, Feb 01, 2012 at 12:24:25PM -0800, Greg Thelen wrote:
>> 1. how to compute per-container pause based on bdi bandwidth, cgroup
>> dirty page usage.
>> 2. how to ensure that writeback will engage even if system and bdi are
>> below respective background dirty ratios, yet a memcg is above its bg
>> dirty limit.
>
> The solution to (1,2) would be something like this:
>
> --- linux-next.orig/mm/page-writeback.c 2012-02-02 14:13:45.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2012-02-02 14:24:11.000000000 +0800
> @@ -654,6 +654,17 @@ static unsigned long bdi_position_ratio(
> pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
>
> + if (memcg) {
> + long long f;
> + x = div_s64((memcg_setpoint - memcg_dirty) << RATELIMIT_CALC_SHIFT,
> + memcg_limit - memcg_setpoint + 1);
> + f = x;
> + f = f * x >> RATELIMIT_CALC_SHIFT;
> + f = f * x >> RATELIMIT_CALC_SHIFT;
> + f += 1 << RATELIMIT_CALC_SHIFT;
> + pos_ratio = pos_ratio * f >> RATELIMIT_CALC_SHIFT;
> + }
> +
> /*
> * We have computed basic pos_ratio above based on global situation. If
> * the bdi is over/under its share of dirty pages, we want to scale
> @@ -1202,6 +1213,8 @@ static void balance_dirty_pages(struct a
> freerun = dirty_freerun_ceiling(dirty_thresh,
> background_thresh);
> if (nr_dirty <= freerun) {
> + if (memcg && memcg_dirty > memcg_freerun)
> + goto start_writeback;
> current->dirty_paused_when = now;
> current->nr_dirtied = 0;
> current->nr_dirtied_pause =
> @@ -1209,6 +1222,7 @@ static void balance_dirty_pages(struct a
> break;
> }
>
> +start_writeback:
> if (unlikely(!writeback_in_progress(bdi)))
> bdi_start_background_writeback(bdi);
>
>
> That makes the minimal change to enforce per-memcg dirty ratio.
> It could result in a less stable control system, but should still
> be able to balance things out.
>
> Thanks,
> Fengguang
Thank you for the quick patch. It looks promising. I can imagine how
this would wake up background writeback. But I am unsure how
background writeback will do anything. It seems like
over_bground_thresh() would not necessarily see system or bdi dirty
usage over respective limits. In previously posted memcg writeback
patches this involved an fs-writeback.c call to
mem_cgroups_over_bground_dirty_thresh() to check for memcg dirty limit
compliance. Do you think we still need such a call out to memcg from
writeback?
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