On Monday, September 05, 2016 11:09:17 AM Mark Johnston wrote:
> On Mon, Sep 05, 2016 at 10:30:24AM -0700, John Baldwin wrote:
> > On Sunday, September 04, 2016 12:29:49 AM Mark Johnston wrote:
> > > Author: markj
> > > Date: Sun Sep 4 00:29:48 2016
> > > New Revision: 305368
> > > URL: https://svnweb.freebsd.org/changeset/base/305368
> > >
> > > Log:
> > > Micro-optimize sleepq_signal().
> > >
> > > Lift a comparison out of the loop that finds the highest-priority thread
> > > on the queue.
> > >
> > > MFC after: 1 week
> >
> > Could this safely use TAILQ_FOREACH_FROM?
>
> Are you suggesting something like this?
>
> besttd = TAILQ_FIRST(&sq->sq_blocked[queue]);
> td = TAILQ_NEXT(besttd, td_slpq);
> TAILQ_FOREACH_FROM(td, &sq->sq_blocked[queue], td_slpq) {
> ...
>
> I think that would work, and it avoids visiting the first element
> unnecessarily when the queue contains more than one element. If the
> queue contains one element, we'd visit it because of
> TAILQ_FOREACH_FROM's surprising behaviour of iterating over the entire
> queue when the listelem is NULL.
I was hoping it was something equivalent to:
for (td = TAILQ_NEXT(besttd); td != NULL; td = TAILQ_NEXT(td)) {
...
}
I guess what we actually want is something like TAILQ_FOREACH_AFTER():
TAILQ_FOREACH_AFTER(td, besttd, &sq->sq_blocked[queue], td_slpq)
that assumed bestttd was not NULL and so avoided the whole "scan the
whole thing".
For now you could perhaps open-code the above loop though?
--
John Baldwin
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