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Date:   Thu, 30 Mar 2023 15:28:52 +0200
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
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On Wed 29-03-23 11:20:21, Marcelo Tosatti wrote:
> On Tue, Mar 28, 2023 at 01:48:02PM +0200, Michal Hocko wrote:
> > 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 ?crit :
> > > > > 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/. 
> 
> Documentation/admin-guide/kernel-per-CPU-kthreads.rst
> 
> SCHED_SOFTIRQ
> -------------
> 
> Do all of the following:
> 
> 1.      Avoid sending scheduler IPIs to the CPU to be de-jittered,
>         for example, ensure that at most one runnable kthread is present
>         on that CPU.  If a thread that expects to run on the de-jittered
>         CPU awakens, the scheduler will send an IPI that can result in
>         a subsequent SCHED_SOFTIRQ.
> 2.      CONFIG_NO_HZ_FULL=y and ensure that the CPU to be de-jittered
>         is marked as an adaptive-ticks CPU using the "nohz_full="
>         boot parameter.  This reduces the number of scheduler-clock
>         interrupts that the de-jittered CPU receives, minimizing its
>         chances of being selected to do the load balancing work that
>         runs in SCHED_SOFTIRQ context.

Quite hidden and easy to miss if you are only aware of isolcpus.

> > Is this a best practice documented anywhere or it just happens to be
> > the case with workloads you deal with?
> 
> Option 2. However Frederic seems interested in matching the exported
> toggles with the known use-cases classes.
> 
> For example, for this guide:
> http://www.comfilewiki.co.kr/en/doku.php?id=comfilepi:improving_real-time_performance:index
> 
> Using nohz_full= would be a benefit (and its not being currently set,
> perhaps due to not knowing all the options?).
> 
> http://www.comfilewiki.co.kr/en/doku.php?id=comfilepi:improving_real-time_performance:index
> 
> 
> AFAIU the workloads for which disabling nohz_full= is a benefit are those
> where the switching between nohz full mode and sched tick enabled mode
> and vice-versa (which involve programming the local timer) happens
> often and is therefore avoidable? For example switching between 1
> runnable task and more than 1 runnable task (and vice versa).

The patch from Frederic is testing for both. You seem to be arguing to
reduce the test and I still do not understand why. Sure some workloads
(following the above) will likely use nohz_full= as well but does it
make sense to build that expectation into the higher level logic? What
is an actual benefit?

-- 
Michal Hocko
SUSE Labs