[Mesa-dev] [PATCH] gallium/util: don't let children of fork & exec inherit our thread affinity

[Michel Dänzer]

On 2018-09-15 3:04 a.m., Marek Olšák wrote:
> On Fri, Sep 14, 2018 at 4:53 AM, Michel Dänzer <michel at daenzer.net> wrote:
>> On 2018-09-13 8:56 p.m., Marek Olšák wrote:
>>
>>> +    * What happens if a driver is unloaded and the app creates a thread?
>>
>> I suppose the child process will likely crash, because the memory
>> address where util_set_full_cpu_affinity was located will either be
>> unmapped or have random other contents?
>>
>> At least in theory, there could also be an issue where the application
>> might have set its own thread affinity before calling fork, which would
>> be clobbered by util_set_full_cpu_affinity in the child process.
>>
>>
>> Last but not least, this doesn't solve the issue of apps such as
>> blender, which spawn their own worker threads after initializing OpenGL
>> (possibly not themselves directly, but via the toolkit or another
>> library; e.g. GTK+4 uses OpenGL by default), inheriting the thread affinity.
>>
>>
>> Due to these issues, setting the thread affinity needs to be disabled by
>> default, and only white-listed for applications where it's known safe
>> and beneficial. This sucks, but I'm afraid that's the reality until
>> there's better API available which allows solving these issues.
> 
> We don't have the bandwidth to maintain whitelists. This will either
> have to be always on or always off.
> 
> On the positive side, only Ryzens with multiple CCXs get all the
> benefits and disadvantages.

In other words, only people who spent relatively large amounts of money
for relatively high-end CPUs will be affected (I'm sure they'll be glad
to know that "common people" aren't affected. ;). Affected applications
will see their performance decreased by a factor of 2-8 (the number of
CCXs in the CPU).

OTOH, only a relatively small number of games will get a significant
benefit from the thread affinity, and the benefit will be smaller than a
factor of 2. This cannot justify risking a performance drop of up to a
factor of 8, no matter how small the risk.

Therefore, the appropriate mechanism is a whitelist.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer