Re: [Qemu-devel] [PATCH 3/3] hw/char/serial: Don't retry on serial_xmit if errno == EPIPE

[Sergio Lopez]

On Fri, Jun 01, 2018 at 12:52:12PM +0100, Daniel P. Berrangé wrote:
> On Thu, May 31, 2018 at 11:52:01AM +0200, Marc-André Lureau wrote:
> > Hi
> > 
> > On Thu, May 31, 2018 at 9:46 AM, Sergio Lopez <address@hidden> wrote:
> > > If writing to the frontend channel failed with EPIPE, don't set up a
> > > retry. EPIPE is not a recoverable error, so trying again is a waste of CPU
> > > cycles.
> > >
> > > If the vCPU writing to the serial device and emulator thread are pinned
> > > to the same pCPU, it can also compromise the stability of the Guest OS,
> > > as both threads will be competing for pCPU's time, with the vCPU
> > > actively polling the serial device and barely giving time to the
> > > emulator thread to make actual progress.
> > > ---
> > >  hw/char/serial.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/hw/char/serial.c b/hw/char/serial.c
> > > index 2c080c9..f26e86b 100644
> > > --- a/hw/char/serial.c
> > > +++ b/hw/char/serial.c
> > > @@ -262,6 +262,7 @@ static void serial_xmit(SerialState *s)
> > >              /* in loopback mode, say that we just received a char */
> > >              serial_receive1(s, &s->tsr, 1);
> > >          } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) != 1 &&
> > > +                   errno != EPIPE &&
> > >                     s->tsr_retry < MAX_XMIT_RETRY) {
> > 
> > Instead of adding explicit handling of EPIPE, shouldn't the code be
> > rewritten to treat -1 return && errno != EAGAIN as fatal?
> 
> Yes, exactly this code is already broken for every single errno
> value, not simply EPIPE. It needs fixing to treat '-1' return code
> correctly instead of retrying everything.

Given that EAGAIN is already taken care of in
chardev/char.c:qemu_chr_write_buffer, in which cases should we retry? Or
should we just drop all the tsr_retry logic?

Sergio.