svn commit: r279539 - head/sys/sys

[John-Mark Gurney]

Julian Elischer wrote this message on Tue, Mar 03, 2015 at 01:20 -0800:
> On 3/2/15 4:55 PM, Neel Natu wrote:
> > Hi Davide,
> >
> > On Mon, Mar 2, 2015 at 12:26 PM, Davide Italiano <davide at freebsd.org> wrote:
> >> On Mon, Mar 2, 2015 at 12:05 PM, John-Mark Gurney <jmg at freebsd.org> wrote:
> >>> Author: jmg
> >>> Date: Mon Mar  2 20:05:16 2015
> >>> New Revision: 279539
> >>> URL: https://svnweb.freebsd.org/changeset/base/279539
> >>>
> >>> Log:
> >>>    give others fair warning that _SPARE2 isn't just cxgb, but used by large
> >>>    number of other subsystems, so you probably don't want _SPARE2..
> >>>
> >>>    ktr needs an overhaul to really only compile in the ones you want,
> >>>    we've long passed the 31 bits it provides..
> >>>
> >> If you really want to do the overhaul (which would be honestly great),
> >> I might consider revamping my work for per-cpu KTR buffer and include
> >> that in the change. Originally it was just an exercise, but then it
> >> evolved and I've been sitting with it in my local tree for a while. I
> >> never had the chutzpah to upstream it because it involves fundamental
> >> changes and breaks compatibility with the old ktrdump(1) format.
> >> A rather outdated (and maybe not completely functional) version of the
> >> patch can be found here:
> >> http://people.freebsd.org/~davide/locking/ktr_percpu.4.diff , which
> >> should give you an high level view of the change.
> >> I can update it to the last version and bring up for review, if
> >> somebody think it might be a sane idea avoiding synchronization on a
> >> single buffer for KTR.
> I think it would be  a problem...
> one of the truely useful things about ktr is that it does use a single 
> buffer.
> this means that you get the true interaction between CPUS.
> Schedgraph relies on this (as one example).

Don't some systems provide a syncronized P-state invariant TSC?  If so,
we can use the TSC clock to tell ordering between cores..

I could definately seeing it be a tunable that lets people force either
single buffer, or PCPU buffer KTR...  Where we know TSC is syncronized,
we default to PCPU and others a single buffer...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."