Re: Linux 3.19-rc3

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 06, 2015 at 05:28:44AM -0800, Kent Overstreet wrote:
> On Tue, Jan 06, 2015 at 02:03:17PM +0100, Peter Zijlstra wrote:
> > Yeah I got that aspect. I'm still trying to get my head around how the
> > wait_event bit would be a natural match though ;-)
> > 
> > Let me stew a bit on that.
> 
> Also, I think it was kind of a surprise to me back in the day how structurally
> similar the wait_event() implementation (circa several years ago, not the
> monstrosity it is now :) and closure_wait_event() turned out - I wasn't aiming
> for that, but that got me thinking there must be something more fundamental
> going on here.

That 'monstrosity' helped reduce the line count significantly, but more
importantly it fixed a fair few inconsistencies across the various
wait_event*() functions. But yes, its a bit of a handful.

Now back to why I don't really like closures for this purpose; the
wait_event*() stuff is really only a wait list, closures are a wait list
+ bits.

So while it makes sense to me to implement closures in terms of
wait_event, the reverse does not make sense to me.

Now you gave a good reason to not use the existing wait list stuff, its
somewhat bloated, and that's fair.

> > That said, the RT people want a simple waitqueue, one that has
> > deterministic behaviour. This is only possibly by removing some of the
> > more obscure waitqueue features and thus also results in a slimmer
> > structure.
> 
> Oh really? That's good to hear.

http://thread.gmane.org/gmane.linux.kernel/1808752 is the last posting
iirc.

> I do like wake_all() being cheaper with the singly linked list, wakups are much
> more common than waiting on things (e.g. the aio code delivering events to the
> ringbuffer, anything that's freeing up resources).
> 
> Been kind of wondering how sane it would be to implement
> finish_wait()/wake_one() with a singly linked list, and maybe preserve some of
> the locklessness. You do fancy lockless stuff too, don't you? Maybe you have
> some ideas :)

Ha! I think I implemented the required nightmare, have a look at:

  fb0527bd5ea9 ("locking/mutexes: Introduce cancelable MCS lock for adaptive spinning")

MCS locks are basically a single linked lockless FIFO queue, however for
the optimistic spinning stuff we needed to be able to abort the lock
op/unlink ourselves.

I'll be the first to admit that that code is somewhat mind bending. I
had to draw quite a few doodles when writing that code :-)