Re: xen-blkback: Scheduled work from previous purge is still busy, cannot purge list

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Please don't drop xen-devel mailing list when replying.

On Thu, Oct 15, 2020 at 01:28:49PM +0200, J. Roeleveld wrote:
> On Thursday, October 15, 2020 12:57:35 PM CEST you wrote:
> > On Tue, Oct 13, 2020 at 07:26:47AM +0200, J. Roeleveld wrote:
> > > Hi All,
> > > 
> > > I am seeing the following message in the "dmesg" output of a driver
> > > domain.
> > > 
> > > [Thu Oct  8 20:57:04 2020] xen-blkback: Scheduled work from previous purge
> > > is still busy, cannot purge list
> > > [Thu Oct  8 20:57:11 2020] xen-blkback: Scheduled work from previous purge
> > > is still busy, cannot purge list
> > > [Thu Oct  8 20:57:44 2020] xen-blkback: Scheduled work from previous purge
> > > is still busy, cannot purge list
> > > [Thu Oct  8 20:57:44 2020] xen-blkback: Scheduled work from previous purge
> > > is still busy, cannot purge list
> > > 
> > > 
> > > Is this something to worry about? Or can I safely ignore this?
> > 
> > What version of the Linux kernel are you running in that driver
> > domain?
> 
> Host:
> Kernel: 5.4.66
> Xen: 4.12.3
> 
> Driver domain:
> Kernel: 5.4.66
> Xen: 4.12.3
> 
> 
> > Is the domain very busy? That might explain the delay in purging
> > grants.
> 
> No, it's generally asleep, been going through the munin-records and can't 
> find 
> any spikes the correlate with the messages either.
> 
> > Also is this an sporadic message, or it's constantly repeating?
> 
> It's sporadic, but occasionally, I get it several times in a row.
> 
> My understanding of the code where this message comes from is far from 
> sufficient. Which means I have no clue what it is actually trying to do.

There's a recurrent worker thread in blkback that will go and purge
unused cache entries after they have expired. This is done to prevent
the cache from growing unbounded.

AFAICT this just means the worker is likely running faster than what
you can proceed, and hence you get another worker run before the old
entries have been removed. Should be safe to ignore, but makes me
wonder if I should add a parameter to tune the periodicity of the
purge work.

Roger.