Re: [Qemu-devel] [PATCH] coroutine: add ./configure --disable-coroutine-pool

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Sep 27, 2013 at 06:49:18PM +0200, Stefan Weil wrote:
> Am 27.09.2013 11:11, schrieb Stefan Hajnoczi:
> > On Fri, Sep 27, 2013 at 07:20:21AM +0200, Stefan Weil wrote:
> >> Am 11.09.2013 16:42, schrieb Stefan Hajnoczi:
> >>> The 'gthread' coroutine backend was written before the freelist (aka
> >>> pool) existed in qemu-coroutine.c.
> >>>
> >>> This means that every thread is expected to exit when its coroutine
> >>> terminates.  It is not possible to reuse threads from a pool.
> >>>
> >>> This patch automatically disables the pool when 'gthread' is used.  This
> >>> allows the 'gthread' backend to work again (for example,
> >>> tests/test-coroutine completes successfully instead of hanging).
> >>>
> >>> I considered implementing thread reuse but I don't want quirks like CPU
> >>> affinity differences due to coroutine threads being recycled.  The
> >>> 'gthread' backend is a reference backend and it's therefore okay to skip
> >>> the pool optimization.
> >>>
> >>> Note this patch also makes it easy to toggle the pool for benchmarking
> >>> purposes:
> >>>
> >>>   ./configure --with-coroutine-backend=ucontext \
> >>>               --disable-coroutine-pool
> >>>
> >>> Reported-by: Gabriel Kerneis <gabriel@kerneis.info>
> >>> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >>> ---
> >>>  configure        | 24 ++++++++++++++++++++++++
> >>>  qemu-coroutine.c | 34 +++++++++++++++++++---------------
> >>>  2 files changed, 43 insertions(+), 15 deletions(-)
> >>>
> >> This patch is important for QEMU 1.5 as well, but needs some
> >> modifications there.
> >> A recent bug report for MinGW shows that the win32 coroutine needs it, too.
> > coroutine-win32.c is designed to support reuse:
> >
> > static void CALLBACK coroutine_trampoline(void *co_)
> > {
> >     Coroutine *co = co_;
> >
> >     while (true) {
> >         co->entry(co->entry_arg);
> >         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
> >     }
> > }
> >
> > We return from qemu_coroutine_switch() when the fiber is reused and
> > simply run another iteration of the while loop.
> >
> > Why do you say win32 coroutines should disable the pool?
> >
> > Stefan
> 
> QEMU MinGW binaries with coroutine pool simply crash, and disabling the
> pool helps
> for the moment until we have a better fix. That's a regression which was
> introduced
> with a patch which added the pool for win32.
> 
> See this discussion thread for more details:
> http://lists.nongnu.org/archive/html/qemu-devel/2013-09/msg04195.html

I'd like to fix the win32 coroutine backend.

In your email you said it can be reproduced under wine on Linux.  I
wasn't able to reproduce it with this command-line:

  $ wine qemu-system-x86_64.exe -m 1024 -vnc :1 -L ../pc-bios -hda ../test.img

Stefan: Do you have a ./configure and QEMU command-line which reproduces
the assertion failure under wine?

Stefan