On Wed, 23 Apr 2014 12:40:58 +1000 NeilBrown <neilb@xxxxxxx> wrote:
> When a loop-back NFS mount is active and the backing device for the
> NFS mount becomes congested, that can impose throttling delays on the
> nfsd threads.
>
> These delays significantly reduce throughput and so the NFS mount
> remains congested.
>
> This results in a live lock and the reduced throughput persists.
>
> This live lock has been found in testing with the 'wait_iff_congested'
> call, and could possibly be caused by the 'congestion_wait' call.
>
> This livelock is similar to the deadlock which justified the
> introduction of PF_LESS_THROTTLE, and the same flag can be used to
> remove this livelock.
>
> To minimise the impact of the change, we still throttle nfsd when the
> filesystem it is writing to is congested, but not when some separate
> filesystem (e.g. the NFS filesystem) is congested.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> mm/vmscan.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a9c74b409681..e011a646de95 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1424,6 +1424,18 @@ putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
> list_splice(&pages_to_free, page_list);
> }
>
> +/* If a kernel thread (such as nfsd for loop-back mounts) services
/*
* If ...
please
> + * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
> + * In that case we should only throttle if the backing device it is
> + * writing to is congested. In other cases it is safe to throttle.
> + */
> +static int current_may_throttle(void)
> +{
> + return !(current->flags & PF_LESS_THROTTLE) ||
> + current->backing_dev_info == NULL ||
> + bdi_write_congested(current->backing_dev_info);
> +}
> +
> /*
> * shrink_inactive_list() is a helper for shrink_zone(). It returns the number
> * of reclaimed pages
> @@ -1552,7 +1564,8 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> * implies that pages are cycling through the LRU faster than
> * they are written so also forcibly stall.
> */
> - if (nr_unqueued_dirty == nr_taken || nr_immediate)
> + if ((nr_unqueued_dirty == nr_taken || nr_immediate)
> + && current_may_throttle())
foo &&
bar
please. As you did in in current_may_throttle().
> congestion_wait(BLK_RW_ASYNC, HZ/10);
> }
>
> @@ -1561,7 +1574,8 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> * is congested. Allow kswapd to continue until it starts encountering
> * unqueued dirty pages or cycling through the LRU too quickly.
> */
> - if (!sc->hibernation_mode && !current_is_kswapd())
> + if (!sc->hibernation_mode && !current_is_kswapd()
> + && current_may_throttle())
ditto
> wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
>
> trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
>
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