INFO: task blocked for more than 120 seconds.

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Under heavy IO load on servers you may see something like:

INFO: task nfsd:2252 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

...probably followed by a call trace that mentions your filesystem, and probably io_schedule and sync_buffer.


This message is not an error, it's telling you that this process has not been scheduled on the CPU at all for 120 seconds, because it was in uninterruptable sleep state. (The code behind this message sits in hung_task.c and was added somewhere around 2.6.30. This is a kernel thread that detects tasks that stays in the D state for a while)


At the same time, 120 real-world seconds is an eternity for the CPU, and most programs, and even many users.

Not being scheduled for that long typically signals resource starvation, usually IO, often some disk API. Which means you usually don't want to silence that message, because you want to find out when and why this happened, and probably avoid it in the future.


The stack trace can help diagnose what it was doing. (which is not so informative of the reason - the named program is often the victim of another one misbehaving, though it is sometimes the culprit)


Reasons include

  • the system is heavily swapping, apparently to the point of trashing, due to memory allocation issues
could be any program
  • the underlying IO system is very slow for some reason
I've seen mentions of this happening in VMs that share disks
  • specific bugs (in kernel code, systemd) have caused this as a side effect






Notes:

  • if it happens constantly your IO system is slower than your IO use
  • can happen to a process that was ioniced into the idle class,
which means it's working as intended, becauseidle-class is meant as an extreme politeness thing. It just indicates something else is doing a consistent bunch of IO right now (for at least 120 seconds)
e.g. updatedb, which may be victim if it were ioniced, cause if not
  • if it happens only nightly, look at your cron jobs
  • a trashing system can cause this, and then it's purely a side effect of program using too more memory than there is RAM
  • being blocked by a desktop-class drive with bad sectors (because they retry for a long while)


  • NFS seems to be a common culprit, probably because it's good at filling the writeback cache, something which implies blocking while writeback happens - which is likely to block various things related to the same filesystem. (verify)
  • if it happens on a fileserver, you may want to consider spreading to more fileservers, or using a parallel filesystem


if your load is fairly sequential, you may get some relief from using the noop io scheduler (instead of cfq) though note that that disables ionice)
if your load is relatively random, upping the queue depth may help