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

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Message-ID: <b7af42dd-2169-5013-93d8-24eb5c37f5bf@linux.intel.com>
Date:   Thu, 27 Apr 2023 17:25:55 +0300 (EEST)
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 Thu, 27 Apr 2023, James Morse wrote:

> Hi Ilpo,
> 
> On 21/03/2023 15:14, Ilpo Järvinen wrote:
> > 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.
> 
> >> 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
> 
> >> +/**
> >> + * 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?
> 
> tick_nohz_full_mask is the list of CPUs we should avoid. This wants to find the first cpu
> set in the domain mask, and clear in tick_nohz_full_mask.
> 
> Where does cpumask_nth_andnot() modify its arguments? Its arguments are const.

Ah, it doesn't, I'm sorry about that.

I think I was trapped by ambiguous English:
 * cpumask_nth_andnot - get the first cpu set in 1st cpumask, and clear in 2nd.
...which can be understood as it clearing it in 2nd.


-- 
 i.