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[Qemu-devel] Re: MIPS, io-thread, icount and wfi
From: |
Marcelo Tosatti |
Subject: |
[Qemu-devel] Re: MIPS, io-thread, icount and wfi |
Date: |
Wed, 19 Jan 2011 15:02:26 -0200 |
User-agent: |
Mutt/1.5.20 (2009-08-17) |
On Tue, Jan 18, 2011 at 11:00:57AM +0100, Jan Kiszka wrote:
> On 2011-01-18 01:19, Edgar E. Iglesias wrote:
> > On Mon, Jan 17, 2011 at 11:03:08AM +0100, Edgar E. Iglesias wrote:
> >> Hi,
> >>
> >> I'm running an io-thread enabled qemu-system-mipsel with icount.
> >> When the guest (linux) goes to sleep through the wait insn (waiting
> >> to be woken up by future timer interrupts), the thing deadlocks.
> >>
> >> IIUC, this is because vm timers are driven by icount, but the CPU is
> >> halted so icount makes no progress and time stands still.
> >>
> >> I've locally disabled vcpu halting when icount is enabled, that
> >> works around my problem but of course makes qemu consume 100% host cpu.
> >>
> >> I don't know why I only see this problem with io-thread builds?
> >> Could be related timing and luck.
> >>
> >> Would be interesting to know if someone has any info on how this was
> >> intended to work (if it was)? And if there are ideas for better
> >> workarounds or fixes that don't disable vcpu halting entirely.
> >
> > Hi,
> >
> > I've found the problem. For some reason io-thread builds use a
> > static timeout for wait loops. The entire chunk of code that
> > makes sure qemu_icount makes forward progress when the CPU's
> > are idle has been ifdef'ed away...
> >
> > This fixes the problem for me, hopefully without affecting
> > io-thread runs without icount.
> >
> > commit 0f4f3a919952500b487b438c5520f07a1c6be35b
> > Author: Edgar E. Iglesias <address@hidden>
> > Date: Tue Jan 18 01:01:57 2011 +0100
> >
> > qemu-timer: Fix timeout calc for io-thread with icount
> >
> > Make sure we always make forward progress with qemu_icount to
> > avoid deadlocks. For io-thread, use the static 1000 timeout
> > only if icount is disabled.
> >
> > Signed-off-by: Edgar E. Iglesias <address@hidden>
> >
> > diff --git a/qemu-timer.c b/qemu-timer.c
> > index 95814af..db1ec49 100644
> > --- a/qemu-timer.c
> > +++ b/qemu-timer.c
> > @@ -110,7 +110,6 @@ static int64_t cpu_get_clock(void)
> > }
> > }
> >
> > -#ifndef CONFIG_IOTHREAD
> > static int64_t qemu_icount_delta(void)
> > {
> > if (!use_icount) {
> > @@ -124,7 +123,6 @@ static int64_t qemu_icount_delta(void)
> > return cpu_get_icount() - cpu_get_clock();
> > }
> > }
> > -#endif
> >
> > /* enable cpu_get_ticks() */
> > void cpu_enable_ticks(void)
> > @@ -1077,9 +1075,17 @@ void quit_timers(void)
> >
> > int qemu_calculate_timeout(void)
> > {
> > -#ifndef CONFIG_IOTHREAD
> > int timeout;
> >
> > +#ifdef CONFIG_IOTHREAD
> > + /* When using icount, making forward progress with qemu_icount when the
> > + guest CPU is idle is critical. We only use the static io-thread
> > timeout
> > + for non icount runs. */
> > + if (!use_icount) {
> > + return 1000;
> > + }
> > +#endif
> > +
> > if (!vm_running)
> > timeout = 5000;
> > else {
> > @@ -1110,8 +1116,5 @@ int qemu_calculate_timeout(void)
> > }
> >
> > return timeout;
> > -#else /* CONFIG_IOTHREAD */
> > - return 1000;
> > -#endif
> > }
> >
> >
> >
>
> This logic and timeout values were imported on iothread merge. And I bet
> at least the timeout value of 1s (vs. 5s) can still be found in
> qemu-kvm. Maybe someone over there can remember the rationales behind
> choosing this value.
>
> Jan
This timeout is for the main select() call. So there is not a lot
of reasoning, how long to wait when there's no activity on the file
descriptors.
- [Qemu-devel] MIPS, io-thread, icount and wfi, Edgar E. Iglesias, 2011/01/17
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi, Edgar E. Iglesias, 2011/01/17
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi, Jan Kiszka, 2011/01/18
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi, Jan Kiszka, 2011/01/18
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi,
Marcelo Tosatti <=
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi, Edgar E. Iglesias, 2011/01/19
- [Qemu-devel] Re: MIPS, io-thread, icount and wfi, Edgar E. Iglesias, 2011/01/22