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

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Date:   Tue, 1 Aug 2017 14:35:03 -0700 (PDT)
From:   Shivappa Vikas <vikas.shivappa@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        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, 1 Aug 2017, Thomas Gleixner wrote:

> 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.

Will fix..

Thanks,
Vikas

>
> Thanks,
>
> 	tglx
>