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

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Message-ID: <ZCLT8iH2ltmQ40my@dhcp22.suse.cz>
Date:   Tue, 28 Mar 2023 13:48:02 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        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

On Mon 27-03-23 07:24:54, Marcelo Tosatti wrote:
> On Fri, Mar 24, 2023 at 11:35:35PM +0100, Frederic Weisbecker wrote:
> > 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:
[...]
> > > > 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?
> 
> Yes, that is the case. But as a note: for the two main types of
> configuration performed (one uses isolcpus=[domain] and the other
> cgroups, for isolating processes) nohz_full= is always set.
> 
> So just testing for nohz_full= would be sufficient (which perhaps would
> make the code simpler).

I do not see any mention about that assumption under Documentation/. Is
this a best practice documented anywhere or it just happens to be the
case with workloads you deal with?

-- 
Michal Hocko
SUSE Labs