Messages in this thread | | | Date | Thu, 25 Apr 2013 12:23:12 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS jitter due to per-CPU kthreads |
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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.
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