Allow faster eventhandler dispatching by keeping pointers to handler lists.

[Ian Lepore]

On Sun, 2017-10-29 at 10:12 -0600, Ian Lepore wrote:
> On Sat, 2017-10-28 at 19:37 -0700, Matt Joras wrote:
> > 
> > On 10/28/2017 19:19, Ian Lepore wrote:
> > > 
> > > 
> > > There has been some talk lately of the kernel eventhandler
> > > mechanism
> > > being inefficient due to holding a lock while walking a global list
> > > doing strcmp() to find the right list of handlers.  I've posted a
> > > phabricator review to alleviate that by allowing high-frequency
> > > events
> > > to pre-define the event list and keep a pointer to it, to avoid the
> > > name lookups.
> > > 
> > > https://reviews.freebsd.org/D12821
> > > 
> > > -- Ian
> > >  
> > Hah, as it happens I just posted a revision with largely the same
> > intention, though I approached the problem a bit differently:
> > https://reviews.freebsd.org/D12814.
> > 
> > Matt
> 
> Actually, it looks like we've done nearly identical work, modulo some
> minor naming differences (that I'm completely agnostic about), and you
> have the EHL_NONEMPTY flag that didn't occur to me until after I
> created the phab review last night.
> 
> The other big difference is that mine still links the fast/static lists
> into the global list-of-lists, to preserve what I call "late loose
> binding" where an event handler can register for events before the
> event provider is present (picture a module that monitors events which
> gets loaded before a module that produces those events).
> 
> It ocurred to me this morning that an optimization for mine would be to
> place any list created by eventhandler_create_list() at the end of the
> global list, on the theory that it will mostly be accessed directly via
> pointer and the items at the head of the global list should be the ones
> more likely to be searched by name.
> 
> -- Ian

Oops, I apparently overlooked _eventhandler_insert_static() in your
changes.  I think if that were changed to first check whether the new
handler list already exists on the global list, our changes really
would be essentially identical.

-- Ian