Re: [PATCH 1/2] sched/isolation: Add cpu_is_isolated() API

From: Michal Hocko <mhocko@suse.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	Frederic Weisbecker <fweisbecker@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Leonardo Bras <leobras@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeelb@google.com>,
	Muchun Song <muchun.song@linux.dev>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH 1/2] sched/isolation: Add cpu_is_isolated() API
Date: Tue, 28 Mar 2023 13:48:02 +0200	[thread overview]
Message-ID: <ZCLT8iH2ltmQ40my@dhcp22.suse.cz> (raw)
In-Reply-To: <ZCFu9jFm/rskeNlM@tpad>

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