On Mar 3, 2015 9:27 AM, "John-Mark Gurney" <j...@funkthat.com> wrote: > > 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 <dav...@freebsd.org> wrote: > > >> On Mon, Mar 2, 2015 at 12:05 PM, John-Mark Gurney <j...@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 A rather outdated (and maybe not completely functional) version of the 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... >
I can't talk about schedgraph because I'm not familiar with the implementation. Can you please elaborate how things will break with a per-CPU buf? I know that everything after Nehalem has a synchronized TSC.Also I've just noticed Matt Dillon introduced a change similar to mine in Dragonfly about 10 years ago. The way they cope with TSC skew is that of resynchronizing the timers periodically, e.g. 1 msec. This is exposed via a SYSCTL that can be disabled on modern processors. Anyhow, I tend to agree this kind of change might be kind of risky as is, and I havent evaluated that on !IA32, which makes the proposal even more problematic. About the double implementation, I think it's not worth our time duplicating the code + the burden of maintaining it. Either single or per-CPU buffer. Given the initial opposition I'm inclined to leave the code as is. -- Davide _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
