[Mesa-dev] [PATCH] RFC: Workaround for pthread_setaffinity_np() seccomp filtering

[Eero Tamminen]

Hi,

On 1.3.2019 11.12, Michel Dänzer wrote:
> On 2019-02-28 8:41 p.m., Marek Olšák wrote:
>>> On Thu, Feb 28, 2019 at 1:37 PM Eero Tamminen <eero.t.tamminen at intel.com>
>>>> Why distro versions of Qemu filter sched_setaffinity() syscall?
>>>
>>> (https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1815889)
>>>
>>> Daniel Berrange (berrange) wrote on 2019-02-27: #19
>>>
>>> "IMHO that mesa change is not valid. It is settings its affinity to
>>> run on all threads which is definitely *NOT* something we want to be
>>> allowed. Management applications want to control which CPUs QEMU runs
>>> on, and as such Mesa should honour the CPU placement that the QEMU
>>> process has.
>>>
>>> This is a great example of why QEMU wants to use seccomp to block
>>> affinity changes to prevent something silently trying to use more CPUs
>>> than are assigned to this QEMU."
>>>
>>
>> Mesa uses thread affinity to optimize memory access performance on some
>> CPUs (see util_pin_thread_to_L3). Other places in Mesa need to restore the
>> original thread affinity for some child threads. Additionally, if games
>> limit the thread affinity, Mesa needs to restore the full thread affinity
>> for some of its child threads.
> 
> The last part sounds like Mesa clearly overstepping its authority.
> 
> 
>> In essence, the thread affinity should only be considered a hint for the
>> kernel for optimal performance. There is no reason to kill the process if
>> it's disallowed. Just ignore the call or modify the thread mask to make it
>> legal.
> 
> The fundamental issue here is that Mesa is using the thread affinity API
> for something else than it's intended for. If there was an API for what
> Mesa wants (encouraging certain sets of threads to run on topologically
> close cores), there should be no need to block that.

Why such process needs to be killed instead the request being masked 
suitably, is there some program that breaks subtly if affinity request 
is masked (and that being worse than the program being killed)?


	- eero