On Thu, Apr 08, 2010 at 09:43:41AM +1000, Dave Chinner wrote:
> [added linux-mm]
Now really added linux-mm.
And there's a patch attached that stops direct reclaim from writing
back dirty pages - it seems to work fine from some rough testing
I've done. Perhaps you might want to give it a spin on a
test box, John?
> On Wed, Apr 07, 2010 at 04:57:11PM +0100, John Berthels wrote:
> > Dave Chinner wrote:
> > >I'm not seeing stacks deeper than about 5.6k on XFS under heavy write
> > >loads. That's nowhere near blowing an 8k stack, so there must be
> > >something special about what you are doing. Can you post the stack
> > >traces that are being generated for the deepest stack generated -
> > >/sys/kernel/debug/tracing/stack_trace should contain it.
> > Appended below. That doesn't seem to reach 8192 but the box it's
> > from has logged:
> >
> > [74649.579386] apache2 used greatest stack depth: 7024 bytes left
> >
> > full dmesg (gzipped) attached.
> > >What is generating the write load?
> >
> > WebDAV PUTs in a modified mogilefs cluster, running
> > apache-mpm-worker (threaded) as the DAV server. The write load is a
> > mix of internet-upload speed writers trickling files up and some
> > local fast replicators copying from elsewhere in the cluster. mpm
> > worker cfg is:
> >
> > ServerLimit 20
> > StartServers 5
> > MaxClients 300
> > MinSpareThreads 25
> > MaxSpareThreads 75
> > ThreadsPerChild 30
> > MaxRequestsPerChild 0
> >
> > File sizes are a mix of small to large (4GB+). Each disk is exported
> > as a mogile device, so it's possible for mogile to pound a single
> > disk with lots of write activity (if the random number generator
> > decides to put lots of files on that device at the same time).
> >
> > We're also seeing occasional slowdowns + high load avg (up to ~300,
> > i.e. MaxClients) with a corresponding number of threads in D state.
> > (This slowdown + high load avg seems to correlate with what would
> > have previously caused a panic on the THREAD_ORDER 1, but not 100%
> > sure).
> >
> > As you can see from the dmesg, this trips the "task xxx blocked for
> > more than 120 seconds." on some of the threads.
> >
> > Don't know if that's related to the stack issue or to be expected
> > under the load.
>
> It looks to be caused by direct memory reclaim trying to clean pages
> with a significant amount of stack already in use. basically there
> is not enough stack space left for the XFS ->writepage path to
> execute in. I can't see any fast fix for this occurring, so you are
> probably best to run with a larger stack for the moment.
>
> As it is, I don't think direct memory reclim should be cleaning
> dirty file pages - it should be leaving that to the writeback
> threads (which are far more efficient at it) or, as a
> last resort, kswapd. Direct memory reclaim is invoked with an
> unknown amount of stack already in use, so there is never any
> guarantee that there is enough stack space left to enter the
> ->writepage path of any filesystem.
>
> MM-folk - have there been any changes recently to writeback of
> pages from direct reclaim that may have caused this,
> or have we just been lucky for a really long time?
>
> Cheers,
>
> Dave.
>
> > Depth Size Location (47 entries)
> > ----- ---- --------
> > 0) 7568 16 mempool_alloc_slab+0x16/0x20
> > 1) 7552 144 mempool_alloc+0x65/0x140
> > 2) 7408 96 get_request+0x124/0x370
> > 3) 7312 144 get_request_wait+0x29/0x1b0
> > 4) 7168 96 __make_request+0x9b/0x490
> > 5) 7072 208 generic_make_request+0x3df/0x4d0
> > 6) 6864 80 submit_bio+0x7c/0x100
> > 7) 6784 96 _xfs_buf_ioapply+0x128/0x2c0 [xfs]
> > 8) 6688 48 xfs_buf_iorequest+0x75/0xd0 [xfs]
> > 9) 6640 32 _xfs_buf_read+0x36/0x70 [xfs]
> > 10) 6608 48 xfs_buf_read+0xda/0x110 [xfs]
> > 11) 6560 80 xfs_trans_read_buf+0x2a7/0x410 [xfs]
> > 12) 6480 80 xfs_btree_read_buf_block+0x5d/0xb0 [xfs]
> > 13) 6400 80 xfs_btree_lookup_get_block+0x84/0xf0 [xfs]
> > 14) 6320 176 xfs_btree_lookup+0xd7/0x490 [xfs]
> > 15) 6144 16 xfs_alloc_lookup_eq+0x19/0x20 [xfs]
> > 16) 6128 96 xfs_alloc_fixup_trees+0xee/0x350 [xfs]
> > 17) 6032 144 xfs_alloc_ag_vextent_near+0x916/0xb30 [xfs]
> > 18) 5888 32 xfs_alloc_ag_vextent+0xe5/0x140 [xfs]
> > 19) 5856 96 xfs_alloc_vextent+0x49f/0x630 [xfs]
> > 20) 5760 160 xfs_bmbt_alloc_block+0xbe/0x1d0 [xfs]
> > 21) 5600 208 xfs_btree_split+0xb3/0x6a0 [xfs]
> > 22) 5392 96 xfs_btree_make_block_unfull+0x151/0x190 [xfs]
> > 23) 5296 224 xfs_btree_insrec+0x39c/0x5b0 [xfs]
> > 24) 5072 128 xfs_btree_insert+0x86/0x180 [xfs]
> > 25) 4944 352 xfs_bmap_add_extent_delay_real+0x41e/0x1660 [xfs]
> > 26) 4592 208 xfs_bmap_add_extent+0x41c/0x450 [xfs]
> > 27) 4384 448 xfs_bmapi+0x982/0x1200 [xfs]
> > 28) 3936 256 xfs_iomap_write_allocate+0x248/0x3c0 [xfs]
> > 29) 3680 208 xfs_iomap+0x3d8/0x410 [xfs]
> > 30) 3472 32 xfs_map_blocks+0x2c/0x30 [xfs]
> > 31) 3440 256 xfs_page_state_convert+0x443/0x730 [xfs]
> > 32) 3184 64 xfs_vm_writepage+0xab/0x160 [xfs]
> > 33) 3120 384 shrink_page_list+0x65e/0x840
> > 34) 2736 528 shrink_zone+0x63f/0xe10
> > 35) 2208 112 do_try_to_free_pages+0xc2/0x3c0
> > 36) 2096 128 try_to_free_pages+0x77/0x80
> > 37) 1968 240 __alloc_pages_nodemask+0x3e4/0x710
> > 38) 1728 48 alloc_pages_current+0x8c/0xe0
> > 39) 1680 16 __get_free_pages+0xe/0x50
> > 40) 1664 48 __pollwait+0xca/0x110
> > 41) 1616 32 unix_poll+0x28/0xc0
> > 42) 1584 16 sock_poll+0x1d/0x20
> > 43) 1568 912 do_select+0x3d6/0x700
> > 44) 656 416 core_sys_select+0x18c/0x2c0
> > 45) 240 112 sys_select+0x4f/0x110
> > 46) 128 128 system_call_fastpath+0x16/0x1b
--
Dave Chinner
david@xxxxxxxxxxxxx
mm: disallow direct reclaim page writeback
From: Dave Chinner <dchinner@xxxxxxxxxx>
When we enter direct reclaim we may have used an arbitrary amount of stack
space, and hence entering the filesystem to do writeback can then lead to
stack overruns.
Writeback from direct reclaim is a bad idea, anyway. The background flusher
threads should 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(). 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.
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
mm/vmscan.c | 13 ++++++-------
1 files changed, 6 insertions(+), 7 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f293372..3c194f4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1829,10 +1829,8 @@ 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 &&
@@ -1874,7 +1872,7 @@ 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,
@@ -1896,7 +1894,7 @@ 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,
@@ -1929,7 +1927,7 @@ 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,
@@ -2570,7 +2568,8 @@ 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,
--
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