Re: [PATCH v5 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

[Robin Jarry]

Waiman Long, Jul 10, 2023 at 21:48:
> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
> disables IBRS when the cstate is 6 or lower. However, there are
> some use cases where a customer may want to use max_cstate=1 to
> lower latency. Such use cases will suffer from the performance
> degradation caused by the enabling of IBRS in the sibling idle thread.
> Add a "ibrs_off" module parameter to force disable IBRS and the
> CPUIDLE_FLAG_IRQ_ENABLE flag if set.
>
> In the case of a Skylake server with max_cstate=1, this new ibrs_off
> option will likely increase the IRQ response latency as IRQ will now
> be disabled.
>
> When running SPECjbb2015 with cstates set to C1 on a Skylake system.
>
> First test when the kernel is booted with: "intel_idle.ibrs_off"
>   max-jOPS = 117828, critical-jOPS = 66047
>
> Then retest when the kernel is booted without the "intel_idle.ibrs_off"
> added.
>   max-jOPS = 116408, critical-jOPS = 58958
>
> That means booting with "intel_idle.ibrs_off" improves performance by:
>   max-jOPS:   1.2%, which could be considered noise range.
>   critical-jOPS: 12%, which is definitely a solid improvement.
>
> The admin-guide/pm/intel_idle.rst file is updated to add a description
> about the new "ibrs_off" module parameter.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

Hi Longman,

Adding back my tag that I had set in v1. Thanks!

Tested-by: Robin Jarry <rjarry@xxxxxxxxxx>