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

[Marcelo Tosatti <mtosatti@redhat.com>]

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:
> > > 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.
> >  
> > 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).