Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim

[Dave Chinner <david@fromorbit.com>]

On Mon, Jul 18, 2011 at 12:22:26PM +1000, Dave Chinner wrote:
> On Fri, Jul 15, 2011 at 12:22:26PM +1000, Dave Chinner wrote:
> > On Thu, Jul 14, 2011 at 01:46:34PM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 14 Jul 2011 00:46:43 -0400
> > > Christoph Hellwig <hch@infradead.org> wrote:
> > > 
> > > > On Thu, Jul 14, 2011 at 10:38:01AM +0900, KAMEZAWA Hiroyuki wrote:
> > > > > > +			/*
> > > > > > +			 * Only kswapd can writeback filesystem pages to
> > > > > > +			 * avoid risk of stack overflow
> > > > > > +			 */
> > > > > > +			if (page_is_file_cache(page) && !current_is_kswapd()) {
> > > > > > +				inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> > > > > > +				goto keep_locked;
> > > > > > +			}
> > > > > > +
> > > > > 
> > > > > 
> > > > > This will cause tons of memcg OOM kill because we have no help of kswapd (now).
> > > > 
> > > > XFS and btrfs already disable writeback from memcg context, as does ext4
> > > > for the typical non-overwrite workloads, and none has fallen apart.
> > > > 
> > > > In fact there's no way we can enable them as the memcg calling contexts
> > > > tend to have massive stack usage.
> > > > 
> > > 
> > > Hmm, XFS/btrfs adds pages to radix-tree in deep stack ?
> > 
> > Here's an example writeback stack trace. Notice how deep it is from
> > the __writepage() call?
> ....
> > 
> > So from ->writepage, there is about 3.5k of stack usage here.  2.5k
> > of that is in XFS, and the worst I've seen is around 4k before
> > getting to the IO subsystem, which in the worst case I've seen
> > consumed 2.5k of stack. IOWs, I've seen stack usage from .writepage
> > down to IO take over 6k of stack space on x86_64....
> 
> BTW, here's a stack frame that indicates swap IO:
> 
> dave@test-4:~$ cat /sys/kernel/debug/tracing/stack_trace
>         Depth    Size   Location    (46 entries)
>         -----    ----   --------
>   0)     5080      40   zone_statistics+0xad/0xc0
>   1)     5040     272   get_page_from_freelist+0x2ad/0x7e0
>   2)     4768     288   __alloc_pages_nodemask+0x133/0x7b0
>   3)     4480      48   kmem_getpages+0x62/0x160
>   4)     4432     112   cache_grow+0x2d1/0x300
>   5)     4320      80   cache_alloc_refill+0x219/0x260
>   6)     4240      64   kmem_cache_alloc+0x182/0x190
>   7)     4176      16   mempool_alloc_slab+0x15/0x20
>   8)     4160     144   mempool_alloc+0x63/0x140
>   9)     4016      16   scsi_sg_alloc+0x4c/0x60
>  10)     4000     112   __sg_alloc_table+0x66/0x140
>  11)     3888      32   scsi_init_sgtable+0x33/0x90
>  12)     3856      48   scsi_init_io+0x31/0xc0
>  13)     3808      32   scsi_setup_fs_cmnd+0x79/0xe0
>  14)     3776     112   sd_prep_fn+0x150/0xa90
>  15)     3664      64   blk_peek_request+0xc7/0x230
>  16)     3600      96   scsi_request_fn+0x68/0x500
>  17)     3504      16   __blk_run_queue+0x1b/0x20
>  18)     3488      96   __make_request+0x2cb/0x310
>  19)     3392     192   generic_make_request+0x26d/0x500
>  20)     3200      96   submit_bio+0x64/0xe0
>  21)     3104      48   swap_writepage+0x83/0xd0
>  22)     3056     112   pageout+0x122/0x2f0
>  23)     2944     192   shrink_page_list+0x458/0x5f0
>  24)     2752     192   shrink_inactive_list+0x1ec/0x410
>  25)     2560     224   shrink_zone+0x468/0x500
>  26)     2336     144   do_try_to_free_pages+0x2b7/0x3f0
>  27)     2192     176   try_to_free_pages+0xa4/0x120
>  28)     2016     288   __alloc_pages_nodemask+0x43f/0x7b0
>  29)     1728      48   kmem_getpages+0x62/0x160
>  30)     1680     128   fallback_alloc+0x192/0x240
>  31)     1552      96   ____cache_alloc_node+0x9a/0x170
>  32)     1456      16   __kmalloc+0x17d/0x200
>  33)     1440     128   kmem_alloc+0x77/0xf0
>  34)     1312     128   xfs_log_commit_cil+0x95/0x3d0
>  35)     1184      96   _xfs_trans_commit+0x1e9/0x2a0
>  36)     1088     208   xfs_create+0x57a/0x640
>  37)      880      96   xfs_vn_mknod+0xa1/0x1b0
>  38)      784      16   xfs_vn_create+0x10/0x20
>  39)      768      64   vfs_create+0xb1/0xe0
>  40)      704      96   do_last+0x5f5/0x770
>  41)      608     144   path_openat+0xd5/0x400
>  42)      464     224   do_filp_open+0x49/0xa0
>  43)      240      96   do_sys_open+0x107/0x1e0
>  44)      144      16   sys_open+0x20/0x30
>  45)      128     128   system_call_fastpath+0x16/0x1b
> 
> 
> That's pretty damn bad. From kmem_alloc to the top of the stack is
> more than 3.5k through the direct reclaim swap IO path. That, to me,
> kind of indicates that even doing swap IO on dirty anonymous pages
> from direct reclaim risks overflowing the 8k stack on x86_64....
> 
> Umm, hold on a second, WTF is my standard create-lots-of-zero-length
> inodes-in-parallel doing swapping? Oh, shit, it's also running about
> 50% slower (50-60k files/s instead of 110-120l files/s)....

It's the memory demand caused by the stack tracer causing the
swapping, and the slowdown is just the overhead of tracer.  2.6.38
doesn't swap very much at all, 2.6.39 swaps a bit more more and
3.0-rc7 is about the same....

IOWs the act of measuring stack usage causes the worst case stack
usage for that workload on 2.6.39 and 3.0-rc7.

Cheers,

Dave
-- 
Dave Chinner
david@fromorbit.com

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