Messages in this thread | | | Date | Thu, 25 Apr 2013 08:52:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS jitter due to per-CPU kthreads |
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On Thu, Apr 25, 2013 at 12:23:12PM +0200, Borislav Petkov wrote: > 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...
Ah! I rewrote the first sentence to read:
Don't use eHCA Infiniband hardware, instead choosing hardware that does not require per-CPU kthreads.
> > > 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>
Thank you, added!
> > ------------------------------------------------------------------------ > > > > 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 :-)
Good change, though, applied.
> > kthreads CPU are not listed here. To reduce OS jitter from non-per-CPU > > s/CPU // > > see above.
Good point, fixed
> > kthreads, bind them to a "housekeeping" CPU dedicated to such work. > > [ … ] > > Ok, it looks good, ship it. > > :-)
Will do! ;-)
Thanx, Paul
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