PERFORCE change 99099 for review

[John Birrell]

On Thu, Jun 15, 2006 at 02:33:24PM -0400, John Baldwin wrote:
> On Wednesday 14 June 2006 16:52, John Birrell wrote:
> > On Tue, Jun 13, 2006 at 09:30:46AM -0400, John Baldwin wrote:
> > > On Monday 12 June 2006 20:47, John Birrell wrote:
> > > > http://perforce.freebsd.org/chv.cgi?CH=99099
> > > > 
> > > > Change 99099 by jb at jb_freebsd2 on 2006/06/13 00:47:28
> > > > 
> > > > 	Add hooks for DTrace to register a function to be called when
> > > > 	the scheduler chooses a new thread to run. This is required for
> > > > 	DTrace vtime.
> > > 
> > > You should probably do this in mi_switch() in kern_synch.c rather than
> > > in this macro.
> > 
> > Looking further at this is seems that the code actually has to be
> > inserted into each scheduler just before the call to cpu_switch because
> > the schedulers are each capable of selecting a new thread right up
> > to that time (see sched_switch).
> 
> Ah, yes, I didn't realize at first you needed to know what the new thread is.

What DTrace is doing is recording a timestamp when the thread starts
running so that it knows the elapsed run-time of the thread. This is used
with the timestamp action so that you can calculate in nanoseconds how long
something took.

--
John Birrell