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

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] qxl: pthread_yield on QXL_IO_NOTIFY_OOM
Date: Tue, 20 Sep 2011 17:14:39 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110904 Red Hat/3.1.14-1.el6_1 Thunderbird/3.1.14

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

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.


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