Re: [PATCH] cpufreq: intel_pstate: Enable HWP IO boost for all servers

[srinivas pandruvada]

On Mon, 2023-03-20 at 18:03 +0100, Giovanni Gherdovich wrote:
> On Thu, 2023-03-02 at 20:14 -0800, Srinivas Pandruvada wrote:
> 

...

Hi Giovanni,

> Hello Srinivas,
> 
> Good catch. We've had HWP IO Boost in the kernel for a while now but
> we
> weren't enabling on most of the modern CPUs... This fixes it.
> 
> One thing though: I've the impression that HWP IO Boost depends on
> having
> per-core p-states -- otherwise you'll be boosting up and down the
> entire machine
> instead of just the one core waking up from IO.
> Enabling the feature on all machines with the ACPI PM server profile
> would
> force it also where per-core p-states aren't available.

This feature only exists on HWP systems. There are no HWP systems
without per core P-states. Here we are enabling for performance and
enterprise servers only.


> 
> Would you agree with this assessment?
> Do I correctly understand the reason why per-core p-states are needed
> here?
This problem with IO will be more pronounced in per-core P-states
systems as it will not be influenced by other cores. But even if non
per-core systems if other cores are idle or lightly loaded, the same
problem can occur. But I don't have data on such systems.

> 
> I remember you mentioned the the dependency on per-core p-states in
> the cover
> letter from the HWP IO Boost submission in 2018
> https://lore.kernel.org/lkml/20180605214242.62156-1-srinivas.pandruvada@xxxxxxxxxxxxxxx/
> 
> I think there's a tradeoff here. Up until this patch, we forgot to
> enable the
> feature on four generations of Intel CPUs. That's a lot of lost
> performance;
> thanks to this patch it won't happen ever again, because nobody will
> have to
> update the model list at every new CPU release.
> 
CPU model updates always gets missed and also misses testing
opportunity.

> On the other side, there may be some penalty for machines that:
> - have HWP
> - don't have per-core p-states
> I don't know how large that interesection is, or how big the penalty.
> 

Thanks,
Srinivas



> 
> Giovanni
>