On Tue, 13 Apr 2010 10:17:58 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> When we enter direct reclaim we may have used an arbitrary amount of stack
> space, and hence enterring the filesystem to do writeback can then lead to
> stack overruns. This problem was recently encountered x86_64 systems with
> 8k stacks running XFS with simple storage configurations.
>
> Writeback from direct reclaim also adversely affects background writeback. The
> background flusher threads should already be taking care of cleaning dirty
> pages, and direct reclaim will kick them if they aren't already doing work. If
> direct reclaim is also calling ->writepage, it will cause the IO patterns from
> the background flusher threads to be upset by LRU-order writeback from
> pageout() which can be effectively random IO. Having competing sources of IO
> trying to clean pages on the same backing device reduces throughput by
> increasing the amount of seeks that the backing device has to do to write back
> the pages.
>
> Hence for direct reclaim we should not allow ->writepages to be entered at all.
> Set up the relevant scan_control structures to enforce this, and prevent
> sc->may_writepage from being set in other places in the direct reclaim path in
> response to other events.
>
> Reported-by: John Berthels <john@xxxxxxxxx>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Hmm. Then, if memoy cgroup is filled by dirty pages, it can't kick writeback
and has to wait for someone else's writeback ?
How long this will take ?
# mount -t cgroup none /cgroup -o memory
# mkdir /cgroup/A
# echo 20M > /cgroup/A/memory.limit_in_bytes
# echo $$ > /cgroup/A/tasks
# dd if=/dev/zero of=./tmpfile bs=4096 count=1000000
Can memcg ask writeback thread to "Wake Up Now! and Write this out!" effectively ?
Thanks,
-Kame
> ---
> mm/vmscan.c | 13 ++++++-------
> 1 files changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e0e5f15..5321ac4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1826,10 +1826,8 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> * writeout. So in laptop mode, write out the whole world.
> */
> writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
> - if (total_scanned > writeback_threshold) {
> + if (total_scanned > writeback_threshold)
> wakeup_flusher_threads(laptop_mode ? 0 : total_scanned);
> - sc->may_writepage = 1;
> - }
>
> /* Take a nap, wait for some writeback to complete */
> if (!sc->hibernation_mode && sc->nr_scanned &&
> @@ -1871,7 +1869,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> {
> struct scan_control sc = {
> .gfp_mask = gfp_mask,
> - .may_writepage = !laptop_mode,
> + .may_writepage = 0,
> .nr_to_reclaim = SWAP_CLUSTER_MAX,
> .may_unmap = 1,
> .may_swap = 1,
> @@ -1893,7 +1891,7 @@ unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
> struct zone *zone, int nid)
> {
> struct scan_control sc = {
> - .may_writepage = !laptop_mode,
> + .may_writepage = 0,
> .may_unmap = 1,
> .may_swap = !noswap,
> .swappiness = swappiness,
> @@ -1926,7 +1924,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> {
> struct zonelist *zonelist;
> struct scan_control sc = {
> - .may_writepage = !laptop_mode,
> + .may_writepage = 0,
> .may_unmap = 1,
> .may_swap = !noswap,
> .nr_to_reclaim = SWAP_CLUSTER_MAX,
> @@ -2567,7 +2565,8 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> struct reclaim_state reclaim_state;
> int priority;
> struct scan_control sc = {
> - .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
> + .may_writepage = (current_is_kswapd() &&
> + (zone_reclaim_mode & RECLAIM_WRITE)),
> .may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
> .may_swap = 1,
> .nr_to_reclaim = max_t(unsigned long, nr_pages,
> --
> 1.6.5
>
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