Re: [PATCH documentation 2/2] kthread: Document ways of reducing OSjitter due to per-CPU kthreads
From: Borislav Petkov
Date: Thu Apr 25 2013 - 06:23:21 EST
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
> > > +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
> > > +non-per-CPU kthreads, bind them to a "housekeeping" CPU that is dedicated
> > s/that/which/
> Good catch -- I chose s/that is//.
> > > +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.
> > > + 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.
> > > + 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.
> > > + 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
> 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@xxxxxxx>
> 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
> kthreads, bind them to a "housekeeping" CPU dedicated to such work.
[ â ]
Ok, it looks good, ship it.
Sent from a fat crate under my desk. Formatting is fine.
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