Re: Linux: balloon_process() causing workqueue lockups?

[Juergen Gross <jgross@xxxxxxxx>]

On 27.08.21 11:01, Jan Beulich wrote:
> Hello,
>
> ballooning down Dom0 by about 16G in one go once in a while causes:
>
> BUG: workqueue lockup - pool cpus=6 node=0 flags=0x0 nice=0 stuck for 64s!
> Showing busy workqueues and worker pools:
> workqueue events: flags=0x0
>    pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=2/256 refcnt=3
>      in-flight: 229:balloon_process
>      pending: cache_reap
> workqueue events_freezable_power_: flags=0x84
>    pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
>      pending: disk_events_workfn
> workqueue mm_percpu_wq: flags=0x8
>    pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
>      pending: vmstat_update
> pool 12: cpus=6 node=0 flags=0x0 nice=0 hung=64s workers=3 idle: 2222 43
>
> I've tried to double check that this isn't related to my IOMMU work
> in the hypervisor, and I'm pretty sure it isn't. Looking at the
> function I see it has a cond_resched(), but aiui this won't help
> with further items in the same workqueue.
>
> Thoughts?
I'm seeing two possible solutions here:

1. After some time (1 second?) in balloon_process() setup a new
   workqueue activity and return (similar to EAGAIN, but without
   increasing the delay).

2. Don't use a workqueue for the ballooning activity, use a kernel
   thread instead.

I have a slight preference for 2, even if the resulting patch will
be larger. 1 is only working around the issue and it is hard to
find a really good timeout value.

I'd be fine to write a patch, but would prefer some feedback which
way to go.


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature