linux-kernel - RE: [PATCH V3 3/4] x86/resctrl: Enable per-thread MBA

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Date:   Thu, 14 May 2020 14:04:27 -0500
From:   Babu Moger <babu.moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "fenghua.yu@...el.com" <fenghua.yu@...el.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tony.luck@...el.com" <tony.luck@...el.com>
Cc:     "kuo-lang.tseng@...el.com" <kuo-lang.tseng@...el.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V3 3/4] x86/resctrl: Enable per-thread MBA



> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@...el.com>
> Sent: Wednesday, May 6, 2020 6:50 PM
> To: tglx@...utronix.de; fenghua.yu@...el.com; bp@...en8.de;
> tony.luck@...el.com
> Cc: kuo-lang.tseng@...el.com; ravi.v.shankar@...el.com; mingo@...hat.com;
> Moger, Babu <Babu.Moger@....com>; hpa@...or.com; x86@...nel.org;
> linux-kernel@...r.kernel.org; Reinette Chatre <reinette.chatre@...el.com>
> Subject: [PATCH V3 3/4] x86/resctrl: Enable per-thread MBA
> 
> From: Fenghua Yu <fenghua.yu@...el.com>
> 
> Current Memory Bandwidth Allocation (MBA) hardware has a limitation:
> all threads on the same core must have the same delay value. If there
> are different delay values across threads on one core, the original
> MBA implementation allocates the max delay value to the core and an
> updated implementation allocates either min or max delay value specified
> by a configuration MSR across threads on the core.
> 
> Newer systems support per-thread MBA such that each thread is allocated
> with its own delay value.
> 
> If per-thread MBA is supported, report "per-thread" in resctrl file
> "info/MB/thread_throttle_mode" to let user applications know memory
> bandwidth is allocated per thread and help them fine tune MBA on thread
> level.
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
> Changes since V2:
> - Fix rST formatting of documentation (resctrl_ui.rst) describing
>   new "thread_throttle_mode" resctrl file.
> - Use boot_cpu_has() instead of static_cpu_has() when determining what
>   to display to user (slow path).
> 
>  Documentation/x86/resctrl_ui.rst       |  3 +++
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 11 +++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/Documentation/x86/resctrl_ui.rst
> b/Documentation/x86/resctrl_ui.rst
> index 861ee2816470..1b066d1aafad 100644
> --- a/Documentation/x86/resctrl_ui.rst
> +++ b/Documentation/x86/resctrl_ui.rst
> @@ -150,6 +150,9 @@ with respect to allocation:
>  		"max":
>  			the smallest percentage is applied
>  			to all threads
> +		"per-thread":
> +			bandwidth percentages are directly applied to
> +			the threads running on the core
> 
>  If RDT monitoring is available there will be an "L3_MON" directory
>  with the following files:
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 6a9408060ac4..c60a3b307f7d 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1038,12 +1038,23 @@ static int max_threshold_occ_show(struct
> kernfs_open_file *of,
>   * with the maximum delay value that from the software interface will be
>   * the minimum of the bandwidth percentages assigned to the hardware threads
>   * sharing the core.
> + *
> + * Some systems (identified by X86_FEATURE_PER_THREAD_MBA enumerated
> via CPUID)
> + * support per-thread MBA. On these systems hardware doesn't apply the
> minimum
> + * or maximum delay value to all threads in a core. Instead, a thread is
> + * allocated with the delay value that is assigned to the thread.
>   */
>  static int rdt_thread_throttle_mode_show(struct kernfs_open_file *of,
>  					 struct seq_file *seq, void *v)
>  {
>  	unsigned int throttle_mode = 0;
> 
> +	if (boot_cpu_has(X86_FEATURE_PER_THREAD_MBA)) {
> +		seq_puts(seq, "per-thread\n");
> +
You probably don't need an extra line here.

> +		return 0;
> +	}
> +
>  	if (mba_cfg_supports_min_max_intel())
>  		throttle_mode = mba_cfg_msr &
> MBA_THROTTLE_MODE_MASK;
> 
> --
> 2.21.0