Re: BTRFS hangs - possibly NFS related?

[Chris Samuel <chris@csamuel.org>]

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On Sun, 25 May 2014 12:42:32 PM kim-btrfs@bluemoose.org.uk wrote:

> Any ideas of what I can do to help debug it...?

Looking at that output it seems that all those blocked processes are in 
congestion_wait() in mm/backing-dev.c, the comments for which say:

/**
 * congestion_wait - wait for a backing_dev to become uncongested
 * @sync: SYNC or ASYNC IO
 * @timeout: timeout in jiffies
 *
 * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
 * write congestion.  If no backing_devs are congested then just wait for the
 * next write to be completed.
 */

The blocked tasks are:

kswapd0
smbd (which correlates with what you've said before)
ShFolders  (is this something local?)
ATA-0 (I suspect a kernel process handling that device)

Interestingly there are no calls to congestion_wait() in fs/btrfs so those 
blocked tasks are blocked accessing other filesystems.

One thing that would be interesting is to see the wchan of processes blocked 
in device wait state when you're in that situation.

Something like this should do it:

ps -eo pid,user,stat,wchan:30,comm | fgrep -w D

Is this system under memory pressure at the time these happen?

All the best,
Chris
- -- 
 Chris Samuel  :  http://www.csamuel.org/  :  Melbourne, VIC

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