Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS jitter due to per-CPU kthreads

[Borislav Petkov <bp@alien8.de>]

On Mon, Apr 22, 2013 at 09:03:29PM -0700, Paul E. McKenney wrote:
> > > +This document lists per-CPU kthreads in the Linux kernel and presents
> > > +options to control OS jitter due to these kthreads.  Note that kthreads
> > 
> > s/due to/which can be caused by/
> 
> Same meaning, but "due to" is probably a bit more arcane.

Yeah, "due to" kinda didn't read right in the context, arcane could be
one way to put it.

> But how about "and presents options to control these kthreads' OS
> jitter"?

Yep.

> > > +that are not per-CPU are not listed here -- to reduce OS jitter from
> > 
> > one too many "that"s:
> > 
> > s/that/which/
> 
> Fair point, but I can shorten it as follows:
> 
> 	Note that non-per-CPU kthreads CPU are not listed here --

that second "CPU" is kinda superfluous...?

> 	to reduce OS jitter from non-per-CPU kthreads, bind them to a
> 	"housekeeping" CPU that is dedicated to such work.

Yep, reads ok, except "that is" but you've removed it in the final
version below.

> > > +non-per-CPU kthreads, bind them to a "housekeeping" CPU that is dedicated
> > 
> > s/that/which/
> 
> Good catch -- I chose s/that is//.

Yep.

> > > +Name: ehca_comp/%u
> > > +Purpose: Periodically process Infiniband-related work.
> > > +To reduce corresponding OS jitter, do any of the following:
> > > +1.	Don't use EHCA Infiniband hardware.  This will prevent these
> > 
> > Sounds like this particular hardware is slow and its IRQ handler/softirq
> > needs a lot of time. Yes, no?
> > 
> > Can we have a reason why people shouldn't use that hw.
> 
> Because it has per-CPU kthreads that can cause OS jitter.  ;-)

Yeah, I stumbled over this specific brand of Infiniband hw. It looks
like this particular Infiniband driver uses per-CPU kthreads and the
others in drivers/infiniband/hw/ don't?

I hope this explains my head-scratching moment here...

> > This sentence keeps repeating; maybe explain the purpose of this doc in
> > the beginning once and drop this sentence in the later sections.
> 
> There are "any of" and "all of" qualifiers.  Also, I cannot count on
> someone reading the document beginning to end.  I would instead expect
> many of them to search for the name of the kthread that is bothering
> them and read only that part.

Ha! Very good point. :-)

> > > +2.	Build with CONFIG_HOTPLUG_CPU=y.  After boot completes, force
> > > +	the CPU offline, then bring it back online.  This forces
> > > +	recurring timers to migrate elsewhere.	If you are concerned
> > 
> > We don't migrate them back to that CPU when we online it again, do we?
> 
> Not unless the CPU it migrated to later is taken offline.  Good point,
> added words to that effect.

Yep, good.

> > > +	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 load balancing,
> > 
> > I don't think there's a "," if the "which... " part refers to the
> > previous "load balancing" and not to the whole sentence.
> 
> Good point -- I can reword to:
> 
> 	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.

Yep.

> > > +	This further reduces the number of scheduler-clock interrupts
> > > +	that the de-jittered CPU receives.
> > 
> > s/that/which/ would suit better here IMHO.
> 
> Fair point, but how about this?
> 
> 	This further reduces the number of scheduler-clock interrupts
> 	received by the de-jittered CPU.

Even better.

> > > +	b.	To the extent possible, keep the CPU out of the kernel
> > > +		when it is non-idle, for example, by avoiding system
> > > +		calls and by forcing both kernel threads and interrupts
> > > +		to execute elsewhere.
> > > +2.	Enable RCU to do its processing remotely via dyntick-idle by
> > > +	doing all of the following:
> > > +	a.	Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
> > > +	b.	Ensure that the CPU goes idle frequently, allowing other
> > 
> > I'm ensuring that by selecting the proper workload which has idle
> > breathers?
> 
> Yep!  Or, equivalently, by adding enough CPUs so that the workload
> has idle breathers.

Yeah, this sentence could be in the text, since we're explaining
everything! :-)

> Thank you for the thorough review and comments!  Please see below for
> an update.

Sure, thank you for writing this up for others to read.

Reviewed-by: Borislav Petkov <bp@suse.de>

> ------------------------------------------------------------------------
> 
> REDUCING OS JITTER DUE TO PER-CPU KTHREADS
> 
> This document lists per-CPU kthreads in the Linux kernel and presents
> options to control these kthreads' OS jitter.  Note that non-per-CPU

s /these kthreads'/their/

Sorry, I can't help it :) I promise I won't read too much in the rest so
as not to beat it to death again :-)

> kthreads CPU are not listed here.  To reduce OS jitter from non-per-CPU

s/CPU //

see above.

> kthreads, bind them to a "housekeeping" CPU dedicated to such work.

[ … ]

Ok, it looks good, ship it.

:-)

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--