Re: [PATCH v4 1/5] sched/deadline: Refer to cpudl.elements atomically

[Byungchul Park <byungchul.park@lge.com>]

On Mon, May 15, 2017 at 09:36:29AM +0100, Juri Lelli wrote:
> Hi,
> 
> On 12/05/17 10:25, Steven Rostedt wrote:
> > On Fri, 12 May 2017 14:48:45 +0900
> > Byungchul Park <byungchul.park@lge.com> 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.
> > 
> > 
> > > 
> > > Current cpudl_find() has problems like,
> > > 
> > >    1. cpudl.elements[0].cpu might not match with cpudl.elements[0].dl.
> > >    2. cpudl.elements[0].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.
> > 
> 
> Right. So, rationale for not taking any lock in the find() path (at the
> risk of getting bogus values) is that we don't want to pay to much in
> terms of contention, when also considering the fact that find_lock_later_
> rq() might then release the rq lock, possibly making the search useless
> (if things change in the meantime anyway). The update path is instead
> guarded by a lock, to ensure consistency.
> 
> Experiments on reasonably big machines (48-cores IIRC) showed that the
> approach was "good enough", so we looked somewhere else to improve
> things (as there are many to improve :). This of course doesn't prevent
> us to look at this again now and see if we need to do something about it.
> 
> Having numbers about introduced overhead and wrong decisions caused by
> the lockless find() path would help a lot understanding what (and can)
> be done.

I see what you say. Agree..

Hm.. Before that, what do you think about my suggestions in my reply to
steven?

> 
> Thanks!
> 
> - Juri