linux-kernel - Re: [PATCH 1/2] sched/isolation: Add cpu_is

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Message-ID: <ZB4lt3IaPWVmn41n@localhost.localdomain>
Date:   Fri, 24 Mar 2023 23:35:35 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <fweisbecker@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leobras@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] sched/isolation: Add cpu_is_isolated() API

Le Sat, Mar 18, 2023 at 09:04:38AM +0100, Michal Hocko a écrit :
> On Fri 17-03-23 15:35:05, Marcelo Tosatti wrote:
> > On Fri, Mar 17, 2023 at 03:33:13PM -0300, Marcelo Tosatti wrote:
> > > On Fri, Mar 17, 2023 at 02:44:47PM +0100, Michal Hocko wrote:
> > > > From: Frederic Weisbecker <frederic@...nel.org>
> > > > 
> > > > Provide this new API to check if a CPU has been isolated either through
> > > > isolcpus= or nohz_full= kernel parameter.
> > > > 
> > > > It aims at avoiding kernel load deemed to be safely spared on CPUs
> > > > running sensitive workload that can't bear any disturbance, such as
> > > > pcp cache draining.
> > > 
> > > Hi Michal,
> > > 
> > > This makes no sense to me.
> > > 
> > > HK_TYPE_DOMAIN is set when isolcpus=domain is configured.
> > > HK_TYPE_TICK is set when nohz_full= is configured.
> > > 
> > > The use-cases i am aware of use either:
> > > 
> > > isolcpus=managed_irq,... nohz_full=
> > > OR
> > > isolcpus=domain,managed_irq,... nohz_full=
> > > 
> > > So what is the point of this function again?
> > > 
> > > Perhaps it made sense along with, but now does not make sense
> > > anymore:
> > > 
> > > Subject: [PATCH 1/2] sched/isolation: Merge individual nohz_full features into a common housekeeping flag
> > > 
> > > The individual isolation features turned on by nohz_full were initially
> > > split in order for each of them to be tunable through cpusets. However
> > > plans have changed in favour of an interface (be it cpusets or sysctl)
> > > grouping all these features to be turned on/off altogether. Then should
> > > the need ever arise, the interface can still be expanded to handle the
> > > individual isolation features.
> > > 
> > > But Michal can just use housekeeping_test_cpu(cpu, HK_TYPE_TICK) and
> > > the convertion of nohz_full features into a common housekeeping flag
> > > can convert that to something else later?
> > 
> > Actually introducing cpu_is_isolated() seems fine, but it can call
> > housekeeping_test_cpu(cpu, HK_TYPE_TICK) AFAICS.
>  
> This is not really my area. Frederic, could you have a look please?

The point is to have a function that tells if either nohz_full= or
isolcpus=[domain] has been passed for the given CPU.

Because I assumed that both would be interested in avoiding that flush
noise, wouldn't it be the case?