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

[Stefan Weil]

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 <address@hidden>
>>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>>> ---
>>>  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

Regards,
Stefan