Re: [PATCH] mm: Avoid possible deadlock caused by too_many_isolated()

[Minchan Kim]

On Fri, Oct 22, 2010 at 1:55 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> Neil find that if too_many_isolated() returns true while performing
> direct reclaim we can end up waiting for other threads to complete their
> direct reclaim.  If those threads are allowed to enter the FS or IO to
> free memory, but this thread is not, then it is possible that those
> threads will be waiting on this thread and so we get a circular
> deadlock.
>
> some task enters direct reclaim with GFP_KERNEL
>  => too_many_isolated() false
>    => vmscan and run into dirty pages
>      => pageout()
>        => take some FS lock
>          => fs/block code does GFP_NOIO allocation
>            => enter direct reclaim again
>              => too_many_isolated() true
>                  => waiting for others to progress, however the other
>                     tasks may be circular waiting for the FS lock..
>
> The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher
> priority than normal ones, by lowering the throttle threshold for the
> latter.
>
> Allowing ~1/8 isolated pages in normal is large enough. For example,
> for a 1GB LRU list, that's ~128MB isolated pages, or 1k blocked tasks
> (each isolates 32 4KB pages), or 64 blocked tasks per logical CPU
> (assuming 16 logical CPUs per NUMA node). So it's not likely some CPU
> goes idle waiting (when it could make progress) because of this limit:
> there are much more sleeping reclaim tasks than the number of CPU, so
> the task may well be blocked by some low level queue/lock anyway.
>
> Now !GFP_IOFS reclaims won't be waiting for GFP_IOFS reclaims to
> progress. They will be blocked only when there are too many concurrent
> !GFP_IOFS reclaims, however that's very unlikely because the IO-less
> direct reclaims is able to progress much more faster, and they won't
> deadlock each other. The threshold is raised high enough for them, so
> that there can be sufficient parallel progress of !GFP_IOFS reclaims.
>
> CC: Torsten Kaiser <just.for.lkml@xxxxxxxxxxxxxx>
> CC: Minchan Kim <minchan.kim@xxxxxxxxx>
> Tested-by: NeilBrown <neilb@xxxxxxx>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>



-- 
Kind regards,
Minchan Kim
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