[igt-dev] [Intel-gfx] [PATCH igt] igt: Add gem_ctx_freq to exercise requesting freq on a ctx

[Tvrtko Ursulin]

On 09/03/2018 13:46, Chris Wilson wrote:
> Exercise some new API that allows applications to request that
> individual contexts are executed within a desired frequency range.
> 
> v2: Split single/continuous set_freq subtests
> v3: Do an up/down ramp for individual freq request, check nothing
> changes after each invalid request
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Paneri, Praveen <praveen.paneri at intel.com>
> Cc: Kamble, Sagar A <sagar.a.kamble at intel.com>
> Cc: Antonio Argenziano <antonio.argenziano at intel.com>
> ---
>   tests/Makefile.am      |   1 +
>   tests/Makefile.sources |   1 +
>   tests/gem_ctx_freq.c   | 648 +++++++++++++++++++++++++++++++++++++++++++++++++
>   tests/meson.build      |   1 +
>   4 files changed, 651 insertions(+)
>   create mode 100644 tests/gem_ctx_freq.c
> 

[snip]

> +static void check_invalid(int fd, uint32_t ctx, uint32_t min, uint32_t max)
> +{
> +	const struct test {
> +		uint32_t min, max;
> +	} tests[] = {
> +		{ min - 50, max - 50 },
> +		{ min - 50, max },
> +		{ min - 50, max + 50 },
> +		{ min, max + 50 },
> +		{ min + 50, max + 50 },
> +
> +		{ min - 50, min - 50 },
> +
> +		{ min - 50, min },
> +		{ min + 50, min },
> +		{ min, min - 50 },
> +
> +		{ max + 50, max },
> +		{ max, max + 50 },
> +		{ max, max - 50 },
> +
> +		{ max + 50, max + 50 },

Is unprivileged "{ max, max }" allowed? In other words pin to max 
frequency by anyone? If so, what happens when all userspace learns about 
this and wants to use it just so to be lower latency than the other guy?

Or even on our integrated graphics, such userspace would unwisely take 
away power budget from the CPU. Or could it even harm itself by 
triggering too much thermal throttling and run slower than if it let the 
balance be controlled by the system?

Or will this be tied with the cgroup work to allow only clients allowed 
by sysadmin to do this?

Regards,

Tvrtko