linux-kernel - Re: [RFC PATCH 23/23] x86/process: Reset hardware history in context switch

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Message-ID: <YzLySV4545F0MKSl@hirez.programming.kicks-ass.net>
Date:   Tue, 27 Sep 2022 14:53:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [RFC PATCH 23/23] x86/process: Reset hardware history in context
 switch

On Fri, Sep 09, 2022 at 04:12:05PM -0700, Ricardo Neri wrote:
> Reset the classification history of the current task when switching to
> the next task. Hardware will start anew the classification of the next
> running task.

Please quantify the cost of this HRESET instruction.

> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 2f314b170c9f..74d8ad83e0b3 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -52,6 +52,7 @@
>  #include <asm/switch_to.h>
>  #include <asm/vm86.h>
>  #include <asm/resctrl.h>
> +#include <asm/hreset.h>
>  #include <asm/proto.h>
>  
>  #include "process.h"
> @@ -214,6 +215,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>  	/* Load the Intel cache allocation PQR MSR. */
>  	resctrl_sched_in();
>  
> +	reset_hardware_history();
> +
>  	return prev_p;
>  }
>  
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 1962008fe743..0b175f30f359 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -53,6 +53,7 @@
>  #include <asm/xen/hypervisor.h>
>  #include <asm/vdso.h>
>  #include <asm/resctrl.h>
> +#include <asm/hreset.h>
>  #include <asm/unistd.h>
>  #include <asm/fsgsbase.h>
>  #ifdef CONFIG_IA32_EMULATION
> @@ -657,6 +658,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>  	/* Load the Intel cache allocation PQR MSR. */
>  	resctrl_sched_in();
>  
> +	reset_hardware_history();
> +
>  	return prev_p;
>  }
>  
> -- 
> 2.25.1
>