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

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Date:   Tue, 16 May 2017 11:32:41 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...il.com, bristot@...hat.com, kernel-team@....com
Subject: Re: [PATCH v4 1/5] sched/deadline: Refer to cpudl.elements atomically

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@....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.
> 
> Yes. That's also whay I hesitated to post this patch..
> 
> > > 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.
> 
> 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?

Thanks,

- Juri