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

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Leonardo Bras <leobras@redhat.com>,
	Frederic Weisbecker <fweisbecker@suse.de>,
	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, Frederic Weisbecker <frederic@kernel.org>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 1/2] sched/isolation: Add cpu_is_isolated() API
Date: Fri, 17 Mar 2023 15:35:05 -0300	[thread overview]
Message-ID: <ZBSy2QZYZRtCNBd8@tpad> (raw)
In-Reply-To: <ZBSyaVk919Fi07Wv@tpad>

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@kernel.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.