linux-kernel - Re: [PATCH 3/3] x86/intel_rdt: Update closid in PQR_ASSOC registers in synchronous mode when changing "cpus"

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Date:   Tue, 15 Nov 2016 15:00:17 +0100 (CET)
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>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Sai Prakhya <sai.praneeth.prakhya@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 3/3] x86/intel_rdt: Update closid in PQR_ASSOC registers
 in synchronous mode when changing "cpus"

On Fri, 11 Nov 2016, Fenghua Yu wrote:
> +/*
> + * MSR_IA32_PQR_ASSOC is scoped per logical CPU, so all updates
> + * are always in thread context.

And this comment tells us what? Nothing useful. It lacks the most important
information why this is safe against a logical CPU switching the MSR right
at this moment during context switch. It's safe because the pqr_switch_to
function is called with interrupts disabled.

> + */
> +static void rdt_update_pqr_assoc_closid(void *v)
> +{
> +	struct intel_pqr_state *state = this_cpu_ptr(&pqr_state);
> +
> +	state->closid = *(int *)v;
> +	wrmsr(MSR_IA32_PQR_ASSOC, state->rmid, state->closid);
> +}
> +
> +static void rdt_update_pqr_assoc(const struct cpumask *cpu_mask, int closid)
> +{
> +	int cpu = get_cpu();
> +
> +	/* Update PQR_ASSOC MSR on this cpu if it's in cpu_mask. */
> +	if (cpumask_test_cpu(cpu, cpu_mask))
> +		rdt_update_pqr_assoc_closid(&closid);
> +	/* Update PQR_ASSOC MSR on the rest of cpus. */
> +	smp_call_function_many(cpu_mask, rdt_update_pqr_assoc_closid,
> +			       &closid, 1);
> +	put_cpu();
> +}
> +
>  static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
>  				   char *buf, size_t nbytes, loff_t off)
>  {
> @@ -238,6 +263,9 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
>  			   &rdtgroup_default.cpu_mask, tmpmask);
>  		for_each_cpu(cpu, tmpmask)
>  			per_cpu(cpu_closid, cpu) = 0;
> +
> +		/* Update PQR_ASSOC registers on the dropped cpus */
> +		rdt_update_pqr_assoc(tmpmask, rdtgroup_default.closid);

Grr. You store the new closid now in the for_each_cpu() loop and in the smp
function call.

>  	}
>  
>  	/*
> @@ -253,6 +281,9 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
>  		}
>  		for_each_cpu(cpu, tmpmask)
>  			per_cpu(cpu_closid, cpu) = rdtgrp->closid;
> +
> +		/* Update PQR_ASSOC registers on the added cpus */
> +		rdt_update_pqr_assoc(tmpmask, rdtgrp->closid);

Ditto.

>  	}
>  
>  	/* Done pushing/pulling - update this group with new mask */
> @@ -783,18 +814,6 @@ static int reset_all_cbms(struct rdt_resource *r)
>  }
>  
>  /*
> - * MSR_IA32_PQR_ASSOC is scoped per logical CPU, so all updates
> - * are always in thread context.
> - */
> -static void rdt_reset_pqr_assoc_closid(void *v)
> -{
> -	struct intel_pqr_state *state = this_cpu_ptr(&pqr_state);
> -
> -	state->closid = 0;
> -	wrmsr(MSR_IA32_PQR_ASSOC, state->rmid, 0);
> -}
> -
> -/*
>   * Forcibly remove all of subdirectories under root.
>   */
>  static void rmdir_all_sub(void)
> @@ -809,13 +828,9 @@ static void rmdir_all_sub(void)
>  		t->closid = 0;
>  	read_unlock(&tasklist_lock);
>  
> -	get_cpu();
> -	/* Reset PQR_ASSOC MSR on this cpu. */
> -	rdt_reset_pqr_assoc_closid(NULL);
> -	/* Reset PQR_ASSOC MSR on the rest of cpus. */
> -	smp_call_function_many(cpu_online_mask, rdt_reset_pqr_assoc_closid,
> -			       NULL, 1);
> -	put_cpu();
> +	/* Reset PQR_ASSOC MSR on all online cpus */
> +	rdt_update_pqr_assoc(cpu_online_mask, 0);

And here you have the extra closid loop later on. Really useful.

Thanks,

	tglx