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Date: Fri, 27 Sep 2013 11:11:07 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130927091107.GA9972@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] coroutine: add ./configure
 --disable-coroutine-pool
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To: Stefan Weil <sw@weilnetz.de>
Cc: Kevin Wolf <kwolf@redhat.com>, gabriel@kerneis.info, qemu-devel@nongnu.org, qemu-stable <qemu-stable@nongnu.org>

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