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

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Message-ID: <alpine.DEB.2.20.1708012142530.2682@nanos>
Date:   Tue, 1 Aug 2017 21:49:09 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>
cc:     vikas.shivappa@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, peterz@...radead.org,
        ravi.v.shankar@...el.com, tony.luck@...el.com,
        fenghua.yu@...el.com, eranian@...gle.com, davidcc@...gle.com,
        ak@...ux.intel.com, reinette.chatre@...el.com
Subject: Re: [PATCH 16/28] x86/intel_rdt: Prepare to add RDT monitor cpus
 file support

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