Re: [Qemu-devel] qxl: pthread_yield on QXL_IO_NOTIFY_OOM

[Gerd Hoffmann <kraxel@redhat.com>]

On 09/20/11 17:00, Jan Kiszka wrote:
> On 2011-09-20 16:48, Gerd Hoffmann wrote:
>> On 09/20/11 16:27, Jan Kiszka wrote:
>>> Hi Gerd,
>>>
>>> can you (or anyone familiar with those bits) comment on pthread_yield()
>>> in ioport_write() of hw/qxl.c?  Which threads are supposed to run this
>>> way?
>>
>> spice server thread.
>>
>>> Can't this relation be expressed explicitly?
>>
>> The thread is created by libspice-server, so we don't have a handle for it.
>
> And also no communication channel to kick?

There is a pipe, and qxl_spice_oom() will kick that way.

I think the yield tries to avoid that kick in case the spice server 
thread is already busy processing stuff.  I'm not fully sure though, the 
code predates /me working on spice.  Each time I look at that piece of 
code I'm asking myself whenever it actually has a positive effect and I 
somehow doubt it.  I never found the time for deeper investigation though.

>> Good question.  I'm tempted to just rip it out and run qxl_spice_oom()
>> unconditionally.  The yield thing is best effort only anyway, there is
>> no guarantee that the spice server thread actually gets scheduled and
>> puts some stuff into the release ring.
>
> And the issue with sleeping is that we stall the vcpu. So just reporting
> oom is likely better.

Agree on that.

cheers,
   Gerd