Re: [PATCH 16/28] x86/intel_rdt: Prepare to add RDT monitor cpus file support

[Thomas Gleixner]

On Tue, 25 Jul 2017, Vikas Shivappa wrote:
>  /*
>   * The cached intel_pqr_state is strictly per CPU and can never be
>   * updated from a remote CPU. Functions which modify the state
> @@ -49,6 +47,8 @@
>   */
>  DEFINE_PER_CPU(struct intel_pqr_state, pqr_state);
>  
> +DEFINE_PER_CPU_READ_MOSTLY(struct intel_pqr_state, rdt_cpu_default);

Cacheline wise this is suboptimal. You have to touch two cachelines on each
context switch (at least for read).

If you make that:

struct intel_pqr_state {
        u32             default_cosid;
        u32             default_rmid;
        u32             cur_cosid;
        u32             cur_rmid;
};

DEFINE_PER_CPU(struct intel_pqr_state, pqr_state);

then it's all together and you spare one cache line.

Thanks,

        tglx