On Mon, Sep 23, 2013 at 11:13:03AM -0400, Steven Rostedt wrote:
> On Mon, 23 Sep 2013 16:54:46 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Sep 23, 2013 at 10:50:17AM -0400, Steven Rostedt wrote:
[ . . . ]
> ?? I'm not sure I understand this. The online_cpus_held++ was there for
> recursion. Can't get_online_cpus() nest? I was thinking it can. If so,
> once the "__cpuhp_writer" is set, we need to do __put_online_cpus() as
> many times as we did a __get_online_cpus(). I don't know where the
> O(nr_tasks) comes from. The ref here was just to account for doing the
> old "get_online_cpus" instead of a srcu_read_lock().
>
> >
> > > static inline void put_online_cpus(void)
> > > {
> > > if (unlikely(current->online_cpus_held)) {
> > > current->online_cpus_held--;
> > > __put_online_cpus();
> > > return;
> > > }
> > >
> > > srcu_read_unlock(&cpuhp_srcu);
> > > }
> >
> > Also, you might not have noticed but, srcu_read_{,un}lock() have an
> > extra idx thing to pass about. That doesn't fit with the hotplug api.
>
> I'll have to look a that, as I'm not exactly sure about the idx thing.
Not a problem, just stuff the idx into some per-task thing. Either
task_struct or taskinfo will work fine.
> > >
> > > Then have the writer simply do:
> > >
> > > __cpuhp_write = current;
> > > synchronize_srcu(&cpuhp_srcu);
> > >
> > > <grab the mutex here>
> >
> > How does that do reader preference?
>
> Well, the point I was trying to do was to let readers go very fast
> (well, with a mb instead of a mutex), and then when the CPU hotplug
> happens, it goes back to the current method.
>
> That is, once we set __cpuhp_write, and then run synchronize_srcu(),
> the system will be in a state that does what it does today (grabbing
> mutexes, and upping refcounts).
>
> I thought the whole point was to speed up the get_online_cpus() when no
> hotplug is happening. This does that, and is rather simple. It only
> gets slow when hotplug is in effect.
Or to put it another way, if the underlying slow-path mutex is
reader-preference, then the whole thing will be reader-preference.
Thanx, Paul
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