On Mon, Apr 24, 2017 at 08:43:18AM -0400, Steven Rostedt wrote:
> On Mon, 24 Apr 2017 10:51:54 +0200
> Peter Zijlstra <pet...@infradead.org> wrote:
>
> > On Fri, Apr 21, 2017 at 10:49:29PM -0400, Steven Rostedt wrote:
> > > When a CPU schedules in a lower priority task and wants to make sure
> > > overloaded CPUs know about it. It increments the rto_loop_next. Then it
> > > does
> > > an atomic_inc_return() on rto_loop_start. If the returned value is not
> > > "1",
> > > then it does atomic_dec() on rt_loop_start and returns. If the value is
> > > "1",
> > > then it will take the rto_lock to synchronize with a possible IPI being
> > > sent
> > > around to the overloaded CPUs.
> >
> > > + start = atomic_inc_return(&rq->rd->rto_loop_start);
> > > + if (start != 1)
> > > + goto out;
> >
> > > +out:
> > > + atomic_dec(&rq->rd->rto_loop_start);
> >
> >
> > Did you just write a very expensive test-and-set trylock?
>
> Probably. I didn't know we had a generic one. Where is it?
>
There isn't. What I was getting at though is that something like:
static inline bool rto_start_trylock(atomic_t *v)
{
int zero = 0;
return atomic_try_cmpxchg(v, &zero, 1);
}
static void rto_start_unlock(atomic_t *v)
{
atomic_set_release(v, 0);
}
Is more: clearer, faster and correct.
Clearer as that it better describes what it does, faster as that you
only have a single atomic, and more correct because it does a RELEASE in
the case we care about.
