linux-kernel - Re: [PATCH v2 10/33] x86/intel_rdt: Implement scheduling support for Intel RDT

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Date:   Thu, 8 Sep 2016 11:53:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     "H. Peter Anvin" <h.peter.anvin@...el.com>,
        Ingo Molnar <mingo@...e.hu>, Tony Luck <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Borislav Petkov <bp@...e.de>,
        Stephane Eranian <eranian@...gle.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Shaohua Li <shli@...com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Sai Prakhya <sai.praneeth.prakhya@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 10/33] x86/intel_rdt: Implement scheduling support
 for Intel RDT

On Thu, 8 Sep 2016, Fenghua Yu wrote:
> +extern struct static_key rdt_enable_key;
> +void __intel_rdt_sched_in(void *dummy);
> +
>  struct clos_cbm_table {
>  	unsigned long cbm;
>  	unsigned int clos_refcnt;
>  };
>  
> +/*
> + * intel_rdt_sched_in() - Writes the task's CLOSid to IA32_PQR_MSR
> + *
> + * Following considerations are made so that this has minimal impact
> + * on scheduler hot path:
> + * - This will stay as no-op unless we are running on an Intel SKU
> + * which supports L3 cache allocation.
> + * - When support is present and enabled, does not do any
> + * IA32_PQR_MSR writes until the user starts really using the feature
> + * ie creates a rdtgroup directory and assigns a cache_mask thats
> + * different from the root rdtgroup's cache_mask.
> + * - Caches the per cpu CLOSid values and does the MSR write only
> + * when a task with a different CLOSid is scheduled in. That
> + * means the task belongs to a different rdtgroup.
> + * - Closids are allocated so that different rdtgroup directories
> + * with same cache_mask gets the same CLOSid. This minimizes CLOSids
> + * used and reduces MSR write frequency.
> + */
> +static inline void intel_rdt_sched_in(void)
> +{
> +	/*
> +	 * Call the schedule in code only when RDT is enabled.
> +	 */
> +	if (static_key_false(&rdt_enable_key))

static_branch_[un]likely() is the proper function to use.

> +		__intel_rdt_sched_in(NULL);
> +
> +void __intel_rdt_sched_in(void *dummy)

What's the purpose of this dummy argument?

> +{
> +	struct intel_pqr_state *state = this_cpu_ptr(&pqr_state);
> +
> +	/*
> +	 * Currently closid is always 0. When  user interface is added,
> +	 * closid will come from user interface.
> +	 */
> +	if (state->closid == 0)
> +		return;
> +
> +	wrmsr(MSR_IA32_PQR_ASSOC, state->rmid, 0);
> +	state->closid = 0;
> +}

Thanks,

	tglx