Re: [patch] mm: fix deferred congestion timeout if preferred zone is not allowed

[Minchan Kim <minchan.kim@gmail.com>]

Hi David,

On Tue, Jan 18, 2011 at 2:09 PM, David Rientjes <rientjes@google.com> wrote:
> Before 0e093d99763e (writeback: do not sleep on the congestion queue if
> there are no congested BDIs or if significant congestion is not being
> encountered in the current zone), preferred_zone was only used for
> statistics and to determine the zoneidx from which to allocate from given
> the type requested.
>
> wait_iff_congested(), though, uses preferred_zone to determine if the
> congestion wait should be deferred because its dirty pages are backed by
> a congested bdi.  This incorrectly defers the timeout and busy loops in
> the page allocator with various cond_resched() calls if preferred_zone is
> not allowed in the current context, usually consuming 100% of a cpu.
>
> This patch resets preferred_zone to an allowed zone in the slowpath if
> the allocation context is constrained by current's cpuset.  It also
> ensures preferred_zone is from the set of allowed nodes when called from
> within direct reclaim; allocations are always constrainted by cpusets
> since the context is always blockable.
>
> Both of these uses of cpuset_current_mems_allowed are protected by
> get_mems_allowed().
> ---
>  mm/page_alloc.c |   12 ++++++++++++
>  mm/vmscan.c     |    3 ++-
>  2 files changed, 14 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2034,6 +2034,18 @@ restart:
>         */
>        alloc_flags = gfp_to_alloc_flags(gfp_mask);
>
> +       /*
> +        * If preferred_zone cannot be allocated from in this context, find the
> +        * first allowable zone instead.
> +        */
> +       if ((alloc_flags & ALLOC_CPUSET) &&
> +           !cpuset_zone_allowed_softwall(preferred_zone, gfp_mask)) {
> +               first_zones_zonelist(zonelist, high_zoneidx,
> +                               &cpuset_current_mems_allowed, &preferred_zone);

This patch is one we need. but I have a nitpick.
I am not familiar with CPUSET so I might be wrong.

I think it could make side effect of statistics of ZVM on
buffered_rmqueue since you intercept and change preferred_zone.
It could make NUMA_HIT instead of NUMA_MISS.
Is it your intention?




-- 
Kind regards,
Minchan Kim

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