Re: [PATCH v4 1/5] sched/deadline: Refer to cpudl.elements atomically
From: Juri Lelli
Date: Tue May 16 2017 - 06:32:49 EST
On 16/05/17 15:52, Byungchul Park wrote:
> On Fri, May 12, 2017 at 10:25:30AM -0400, Steven Rostedt wrote:
> > On Fri, 12 May 2017 14:48:45 +0900
> > Byungchul Park <byungchul.park@xxxxxxx> wrote:
> > > cpudl.elements is an instance that should be protected with a spin lock.
> > > Without it, the code would be insane.
> > And how much contention will this add? Spin locks in the scheduler code
> > that are shared among a domain can cause huge latency. This was why I
> > worked hard not to add any in the cpupri code.
> Yes. That's also whay I hesitated to post this patch..
> > > Current cpudl_find() has problems like,
> > >
> > > 1. cpudl.elements.cpu might not match with cpudl.elements.dl.
> > > 2. cpudl.elements.dl(u64) might not be referred atomically.
> > > 3. Two cpudl_maximum()s might return different values.
> > > 4. It's just insane.
> > And lockless algorithms usually are insane. But locks come with a huge
> > cost. What happens when we have 32 core domains. This can cause
> > tremendous contention and makes the entire cpu priority for deadlines
> > useless. Might as well rip out the code.
> I think it would be better if we, even keeping it lockless,
> 1. make cp->elements.cpu referred once than twice,
> 2. add retry logic in order to match elements.cpu with its dl,
> 3. add retry logic for the u64 variable to be read atomically,
> So what do you think about the suggestions? Of course it does not solve
> the problems perfectly though.. Or do you think it's not worth?
Not sure, but if we are going to retry a lot it might be better off to
put proper locking instead? We could also simply bail out when we notice
that something is changed under our feet. I'd say (again :) we might
first want to understand (with some numbers) how bad the problem is and
then fix it. I guess numbers might also help us to better understand
what the best fix is?