linux-kernel - Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any

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Date:   Tue, 21 Mar 2023 17:14:49 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     James Morse <james.morse@....com>
cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        carl@...amperecomputing.com, lcherian@...vell.com,
        bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
        xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com
Subject: Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping()
 for limbo/overflow

On Mon, 20 Mar 2023, James Morse wrote:

> The limbo and overflow code picks a CPU to use from the domain's list
> of online CPUs. Work is then scheduled on these CPUs to maintain
> the limbo list and any counters that may overflow.
> 
> cpumask_any() may pick a CPU that is marked nohz_full, which will
> either penalise the work that CPU was dedicated to, or delay the
> processing of limbo list or counters that may overflow. Perhaps
> indefinitely. Delaying the overflow handling will skew the bandwidth
> values calculated by mba_sc, which expects to be called once a second.
> 
> Add cpumask_any_housekeeping() as a replacement for cpumask_any()
> that prefers housekeeping CPUs. This helper will still return
> a nohz_full CPU if that is the only option. The CPU to use is
> re-evaluated each time the limbo/overflow work runs. This ensures
> the work will move off a nohz_full CPU once a houskeeping CPU is

housekeeping

> available.
> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
>  arch/x86/kernel/cpu/resctrl/internal.h | 23 +++++++++++++++++++++++
>  arch/x86/kernel/cpu/resctrl/monitor.c  | 17 ++++++++++++-----
>  include/linux/tick.h                   |  3 ++-
>  3 files changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 87545e4beb70..0b5fd5a0cda2 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -7,6 +7,7 @@
>  #include <linux/kernfs.h>
>  #include <linux/fs_context.h>
>  #include <linux/jump_label.h>
> +#include <linux/tick.h>
>  #include <asm/resctrl.h>
>  
>  #define L3_QOS_CDP_ENABLE		0x01ULL
> @@ -55,6 +56,28 @@
>  /* Max event bits supported */
>  #define MAX_EVT_CONFIG_BITS		GENMASK(6, 0)
>  
> +/**
> + * cpumask_any_housekeeping() - Chose any cpu in @mask, preferring those that
> + *			        aren't marked nohz_full
> + * @mask:	The mask to pick a CPU from.
> + *
> + * Returns a CPU in @mask. If there are houskeeping CPUs that don't use
> + * nohz_full, these are preferred.
> + */
> +static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
> +{
> +	int cpu, hk_cpu;
> +
> +	cpu = cpumask_any(mask);
> +	if (tick_nohz_full_cpu(cpu)) {
> +		hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);

Why cpumask_nth_and() is not enough here? ..._andnot() seems to alter 
tick_nohz_full_mask which doesn't seem desirable?


-- 
 i.