Re: page allocation stall in kernel 4.9 when copying files from one btrfs hdd to another

[admin]

Hi,

The source is a software raid 5 (md) of 4x 4TB Western Digital RE4 disks. The destinations is a hardware raid 5 enclosure containing 4x 8TB Seagate Archival disks connected using e-sata. 

I am currently trying Duncans suggestions. With them, the page allocation stall doesn't seem to appear and overall system responsiveness is also much better during copying.

Thanks,
David Arendt


Xin Zhou â Thu., 15. December 2016 0:24
> Hi,
> 
> The dirty data is in large amount, probably unable to commit to disk.
> And this seems to happen when copying from 7200rpm to 5600rpm disks, according to previous post.
> 
> Probably the I/Os are buffered and pending, unable to get finished in-time.
> It might be helpful to know if this only happens for specific types of 5600 rpm disks?
> 
> And are these disks on RAID groups? Thanks.
> Xin
> Â
> Â
> 
> Sent:ÂWednesday, December 14, 2016 at 3:38 AM
> From:Âadmin <admin@xxxxxxxxx>
> To:Â"Michal Hocko" <mhocko@xxxxxxxxxx>
> Cc:Âlinux-btrfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, "David Sterba" <dsterba@xxxxxxx>, "Chris Mason" <clm@xxxxxx>
> Subject:ÂRe: page allocation stall in kernel 4.9 when copying files from one btrfs hdd to another
> Hi,
> 
> I verified the log files and see no prior oom killer invocation. Unfortunately the machine has been rebooted since. Next time it happens, I will also look in dmesg.
> 
> Thanks,
> David Arendt
> 
> 
> Michal Hocko â Wed., 14. December 2016 11:31
> > Btw. the stall should be preceded by the OOM killer invocation. Could
> > you share the OOM report please. I am asking because such an OOM killer
> > would be clearly pre-mature as per your meminfo. I am trying to change
> > that code and seeing your numbers might help me.
> >
> > Thanks!
> >
> > On Wed 14-12-16 11:17:43, Michal Hocko wrote:
> > > On Tue 13-12-16 18:11:01, David Arendt wrote:
> > > > Hi,
> > > >
> > > > I receive the following page allocation stall while copying lots of
> > > > large files from one btrfs hdd to another.
> > > >
> > > > Dec 13 13:04:29 server kernel: kworker/u16:8: page allocation stalls for 12260ms, order:0, mode:0x2400840(GFP_NOFS|__GFP_NOFAIL)
> > > > Dec 13 13:04:29 server kernel: CPU: 0 PID: 24959 Comm: kworker/u16:8 Tainted: P O 4.9.0 #1
> > > [...]
> > > > Dec 13 13:04:29 server kernel: Call Trace:
> > > > Dec 13 13:04:29 server kernel: [<ffffffff813f3a59>] ? dump_stack+0x46/0x5d
> > > > Dec 13 13:04:29 server kernel: [<ffffffff81114fc1>] ? warn_alloc+0x111/0x130
> > > > Dec 13 13:04:33 server kernel: [<ffffffff81115c38>] ? __alloc_pages_nodemask+0xbe8/0xd30
> > > > Dec 13 13:04:33 server kernel: [<ffffffff8110de74>] ? pagecache_get_page+0xe4/0x230
> > > > Dec 13 13:04:33 server kernel: [<ffffffff81323a8b>] ? alloc_extent_buffer+0x10b/0x400
> > > > Dec 13 13:04:33 server kernel: [<ffffffff812ef8c5>] ? btrfs_alloc_tree_block+0x125/0x560
> > >
> > > OK, so this is
> > > find_or_create_page(mapping, index, GFP_NOFS|__GFP_NOFAIL)
> > >
> > > The main question is whether this really needs to be NOFS request...
> > >
> > > > Dec 13 13:04:33 server kernel: [<ffffffff8132442f>] ? read_extent_buffer_pages+0x21f/0x280
> > > > Dec 13 13:04:33 server kernel: [<ffffffff812d81f1>] ? __btrfs_cow_block+0x141/0x580
> > > > Dec 13 13:04:33 server kernel: [<ffffffff812d87b0>] ? btrfs_cow_block+0x100/0x150
> > > > Dec 13 13:04:33 server kernel: [<ffffffff812dc1d9>] ? btrfs_search_slot+0x1e9/0x9c0
> > > > Dec 13 13:04:33 server kernel: [<ffffffff8131ead2>] ? __set_extent_bit+0x512/0x550
> > > > Dec 13 13:04:33 server kernel: [<ffffffff812e1ab5>] ? lookup_inline_extent_backref+0xf5/0x5e0
> > > > Dec 13 13:04:34 server kernel: [<ffffffff8131f0a4>] ? set_extent_bit+0x24/0x30
> > > > Dec 13 13:04:34 server kernel: [<ffffffff812e4334>] ? update_block_group.isra.34+0x114/0x380
> > > > Dec 13 13:04:34 server kernel: [<ffffffff812e4694>] ? __btrfs_free_extent.isra.35+0xf4/0xd20
> > > > Dec 13 13:04:34 server kernel: [<ffffffff8134d561>] ? btrfs_merge_delayed_refs+0x61/0x5d0
> > > > Dec 13 13:04:34 server kernel: [<ffffffff812e8bd2>] ? __btrfs_run_delayed_refs+0x902/0x10a0
> > > > Dec 13 13:04:34 server kernel: [<ffffffff812ec0f0>] ? btrfs_run_delayed_refs+0x90/0x2a0
> > > > Dec 13 13:04:34 server kernel: [<ffffffff812ec384>] ? delayed_ref_async_start+0x84/0xa0
> > >
> > > What would cause the reclaim recursion?
> > >
> > > > Dec 13 13:04:34 server kernel: Mem-Info:
> > > > Dec 13 13:04:34 server kernel: active_anon:20 inactive_anon:34
> > > > isolated_anon:0\x0a active_file:7370032 inactive_file:450105
> > > > isolated_file:320\x0a unevictable:0 dirty:522748 writeback:189
> > > > unstable:0\x0a slab_reclaimable:178255 slab_unreclaimable:124617\x0a
> > > > mapped:4236 shmem:0 pagetables:1163 bounce:0\x0a free:38224 free_pcp:241
> > > > free_cma:0
> > >
> > > This speaks for itself. There is a lot of dirty data, basically no
> > > anonymous memory and GFP_NOFS cannot do much to reclaim obviously. This
> > > is either a configuraion bug as somebody noted down the thread (setting
> > > the dirty_ratio) or suboptimality of the btrfs code which might request
> > > NOFS even though it is not strictly necessary. This would be more for
> > > btrfs developers.
> > > --
> > > Michal Hocko
> > > SUSE Labs
> >
> > --
> > Michal Hocko
> > SUSE Labs
> --
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