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SubjectRe: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a_ops->writepages() where possible
On Tue, Jun 15, 2010 at 03:51:34PM +0100, Mel Gorman wrote:
> kswapd does end up freeing a lot of memory in response to lumpy reclaim
> because it also tries to restore watermarks for a high-order page. This
> is disruptive to the system and something I'm going to revisit but it's
> a separate topic for another discussion. I can see why transparent
> hugepage support would not want this disruptive effect to occur where as
> it might make sense when resizing the hugepage pool.

on a related topic, I also had to nuke lumpy reclaim, it's pointless
with mem compaction and it halts the system and makes it unusable
under all normal loads unless allocations are run like hugetlbfs does
(just all at once at app startup and never again, so the hang is
limited to the first minute when app starts). With a dynamic approach
like THP systems becomes unusable. Nothing should fail when large
order allocation fails (I mean the large order that activates lumpy
reclaims) so there's no point to grind the system to unusable state in
order to generate those large order pages, considering lumpy reclaim
effectives is next to irrelevant compared to compaction, and in turn
not worth it.

> Depth Size Location (49 entries)
> ----- ---- --------
> 0) 5064 304 get_page_from_freelist+0x2e4/0x722
> 1) 4760 240 __alloc_pages_nodemask+0x15f/0x6a7
> 2) 4520 48 kmem_getpages+0x61/0x12c
> 3) 4472 96 cache_grow+0xca/0x272
> 4) 4376 80 cache_alloc_refill+0x1d4/0x226
> 5) 4296 64 kmem_cache_alloc+0x129/0x1bc
> 6) 4232 16 mempool_alloc_slab+0x16/0x18
> 7) 4216 144 mempool_alloc+0x56/0x104
> 8) 4072 16 scsi_sg_alloc+0x48/0x4a [scsi_mod]
> 9) 4056 96 __sg_alloc_table+0x58/0xf8
> 10) 3960 32 scsi_init_sgtable+0x37/0x8f [scsi_mod]
> 11) 3928 32 scsi_init_io+0x24/0xce [scsi_mod]
> 12) 3896 48 scsi_setup_fs_cmnd+0xbc/0xc4 [scsi_mod]
> 13) 3848 144 sd_prep_fn+0x1d3/0xc13 [sd_mod]
> 14) 3704 64 blk_peek_request+0xe2/0x1a6
> 15) 3640 96 scsi_request_fn+0x87/0x522 [scsi_mod]
> 16) 3544 32 __blk_run_queue+0x88/0x14b
> 17) 3512 48 elv_insert+0xb7/0x254
> 18) 3464 48 __elv_add_request+0x9f/0xa7
> 19) 3416 128 __make_request+0x3f4/0x476
> 20) 3288 192 generic_make_request+0x332/0x3a4
> 21) 3096 64 submit_bio+0xc4/0xcd
> 22) 3032 80 _xfs_buf_ioapply+0x222/0x252 [xfs]
> 23) 2952 48 xfs_buf_iorequest+0x84/0xa1 [xfs]
> 24) 2904 32 xlog_bdstrat+0x47/0x4d [xfs]
> 25) 2872 64 xlog_sync+0x21a/0x329 [xfs]
> 26) 2808 48 xlog_state_release_iclog+0x9b/0xa8 [xfs]
> 27) 2760 176 xlog_write+0x356/0x506 [xfs]
> 28) 2584 96 xfs_log_write+0x5a/0x86 [xfs]
> 29) 2488 368 xfs_trans_commit_iclog+0x165/0x2c3 [xfs]
> 30) 2120 80 _xfs_trans_commit+0xd8/0x20d [xfs]
> 31) 2040 240 xfs_iomap_write_allocate+0x247/0x336 [xfs]
> 32) 1800 144 xfs_iomap+0x31a/0x345 [xfs]
> 33) 1656 48 xfs_map_blocks+0x3c/0x40 [xfs]
> 34) 1608 256 xfs_page_state_convert+0x2c4/0x597 [xfs]
> 35) 1352 64 xfs_vm_writepage+0xf5/0x12f [xfs]
> 36) 1288 32 __writepage+0x17/0x34
> 37) 1256 288 write_cache_pages+0x1f3/0x2f8
> 38) 968 16 generic_writepages+0x24/0x2a
> 39) 952 64 xfs_vm_writepages+0x4f/0x5c [xfs]
> 40) 888 16 do_writepages+0x21/0x2a
> 41) 872 48 writeback_single_inode+0xd8/0x2f4
> 42) 824 112 writeback_inodes_wb+0x41a/0x51e
> 43) 712 176 wb_writeback+0x13d/0x1b7
> 44) 536 128 wb_do_writeback+0x150/0x167
> 45) 408 80 bdi_writeback_task+0x43/0x117
> 46) 328 48 bdi_start_fn+0x76/0xd5
> 47) 280 96 kthread+0x82/0x8a
> 48) 184 184 kernel_thread_helper+0x4/0x10
>
> XFS as you can see is quite deep there. Now consider if
> get_page_from_freelist() there had entered direct reclaim and then tried
> to writeback a page. That's the problem that is being worried about.

As said in other email this can't be a problem, 5k is very ok there
and there's zero risk as writepage can't reenter itself or fs would
lockup.

This even the above trace, already shows that 5k is used just for xfs
writepage itself, so that means generic kernel code can't exceed 3k, I
agree it's too risky (at least with xfs, dunno if ext4 also eats ~5k
just for writepage + bio).

> I also haven't been able to trigger a new OOM as a result of the patch
> but maybe I'm missing something. To trigger an OOM, the bulk of the LRU

Well you're throttling and waiting I/O from the kernel thread, so it
should be fully safe and zero risk for OOM regressions, agreed!

But if we make changes to tackle this "risk", I prefer if we allow to
remove the PF_MEMALLOC in ext4_write_inode too.. and we instead allow
it to run when __GFP_FS|__GFP_IO is set.

> I hadn't posted them because they had been posted previously and I
> didn't think they were that interesting as such because it wasn't being
> disputed.

No problem, I didn't notice those prev reports, the links you posted
have been handy to find them more quickly ;), that's surely more than
enough, thanks!


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