RE: Serial stdio overwhelming interrupts

[Hollis Blanchard <hollisb@us.ibm.com>]

On Fri, 2008-08-15 at 10:08 +0800, Liu Yu wrote:
> > -----Original Message-----
> > From: Christian Ehrhardt [mailto:ehrhardt@linux.vnet.ibm.com] 
> > Sent: Wednesday, August 13, 2008 8:47 PM
> > To: Liu Yu
> > Cc: kvm-ppc@vger.kernel.org
> > Subject: Re: Serial stdio overwhelming interrupts
> > 
> > Liu Yu wrote:
> > > Hi guys,
> > >
> > > In my kvm machine, I use serial as its default stdio.
> > > But when display a number of characters, e.g. cat a big ascii file, 
> > > the serial always hangs as encount overwhelming interrupts, 
> > and then output a kernel log "serial8250: too much work for irq 42".
> > >
> > > I found that the serial in qemu is primitive so that it 
> > generate a interrupt on every single character. 
> > > This make it easier to reach the threshold of reporting 
> > "too much work".
> > >
> > > I had to use the patch below to get this issue walked arroud.
> > >
> > > Have you guys ever met the serial hanging problem before?
> > >   
> > 
> > We had that some times in the beginning while we brought up 
> > interrupts.
> > But since that time it did never occur again and I thought we 
> > fixed it.
> > Maybe your use case of catting a file to it is stressing it 
> > now to a new extend triggering it again.
> > 
> 
> I found this.
> http://www.archivum.info/qemu-devel@nongnu.org/2008-02/msg00169.html
> I don’t know how it is going now. Because I still work on the branch kvm-70rc1 of qemu
> Maybe it has been solved.

AFAIK it has not been solved, and neither Linux nor qemu developers plan
to change their behavior. In the URL above, a kernel developer suggests
changing qemu's behavior.

The problem is that the Linux driver reads serial data in a loop but
gives up after a fixed number (256) of iterations. I'm not sure why the
kernel does that, since I think interrupts are enabled while that
happens.

As a workaround you could use telnet or ssh to cat your large files.

You're welcome to reopen the discussion on the LKML and KVM lists.

-- 
Hollis Blanchard
IBM Linux Technology Center