writeout stalls in current -git

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Thu, Nov 01, 2007 at 07:20:51PM +0100, Torsten Kaiser wrote:
> On 11/1/07, Fengguang Wu <wfg@mail.ustc.edu.cn> wrote:
> > On Wed, Oct 31, 2007 at 04:22:10PM +0100, Torsten Kaiser wrote:
> > > Since 2.6.23-mm1 I also experience strange hangs during heavy writeouts.
> > > Each time I noticed this I was using emerge (package util from the
> > > gentoo distribution) to install/upgrade a package. The last step,
> > > where this hang occurred, is moving the prepared files from a tmpfs
> > > partion to the main xfs filesystem.
> > > The hangs where not fatal, after a few second everything resumed
> > > normal, so I was not able to capture a good image of what was
> > > happening.
> >
> > Thank you for the detailed report.
> >
> > How severe was the hangs? Only writeouts stalled, all apps stalled, or
> > cannot type and run new commands?
> 
> Only writeout stalled. The emerge that was moving the files hung, but
> everything else worked normaly.
> I was able to run new commands, like coping the /proc/meminfo.

But you mentioned in the next mail that `watch cat /proc/meminfo`
could also be blocked for some time - I guess in the same time emerge
was stalled?

> [snip]
> > > After this SysRq+W writeback resumed again. Possible that writing
> > > above into the syslog triggered that.
> >
> > Maybe. Are the log files on another disk/partition?
> 
> No, everything was going to /
> 
> What might be interesting is, that doing cat /proc/meminfo
> >~/stall/meminfo did not resume the writeback. So there might some
> threshold that only was broken with the additional write from
> syslog-ng. Or syslog-ng does some flushing, I dont now. (I'm using the

Have you tried explicit `sync`? ;-)

> syslog-ng package from gentoo:
> http://www.balabit.com/products/syslog_ng/ , version 2.0.5)
> 
> > > The source tmpfs is mounted with any special parameters, but the
> > > target xfs filesystem resides on a dm-crypt device that is on top a 3
> > > disk RAID5 md.
> > > During the hang all CPUs where idle.
> >
> > No iowaits? ;-)
> 
> No, I have a KSysGuard in my taskbar that showed no activity at all.
> 
> OK, the subject does not match for my case, but there was also a tmpfs
> involved. And I found no thread with stalls on xfs. :-)

Do you mean it is actually related with tmpfs?

> > > The system is x86_64 with CONFIG_NO_HZ=y, but was still receiving ~330
> > > interrupts per second because of the bttv driver. (But I was not using
> > > that device at this time.)
> > >
> > > I'm willing to test patches or more provide more information, but lack
> > > a good testcase to trigger this on demand.
> >
> > Thank you. Maybe we can start by the applied debug patch :-)
> 
> Will applied it and try to recreate this.
> 
> Thanks for looking into it.

Thank you for the rich information, too :-)

Fengguang