Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpu instead of current one

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2012-11-27 at 19:18 +0530, Viresh Kumar wrote:
> On 27 November 2012 18:56, Steven Rostedt <rostedt@goodmis.org> wrote:
> > A couple of things. The sched_select_cpu() is not cheap. It has a double
> > loop of domains/cpus looking for a non idle cpu. If we have 1024 CPUs,
> > and we are CPU 1023 and all other CPUs happen to be idle, we could be
> > searching 1023 CPUs before we come up with our own.
> 
> Not sure if you missed the first check sched_select_cpu()
> 
> +int sched_select_cpu(unsigned int sd_flags)
> +{
> +       /* If Current cpu isn't idle, don't migrate anything */
> +       if (!idle_cpu(cpu))
> +               return cpu;
> 
> We aren't going to search if we aren't idle.

OK, we are idle, but CPU 1022 isn't. We still need a large search. But,
heh we are idle we can spin. But then why go through this in the first
place ;-)


> 
> > Also, I really don't like this as a default behavior. It seems that this
> > solution is for a very special case, and this can become very intrusive
> > for the normal case.
> 
> We tried with an KCONFIG option for it, which Tejun rejected.

Yeah, I saw that. I don't like adding KCONFIG options either. Best is to
get something working that doesn't add any regressions. If you can get
this to work without making *any* regressions in the normal case than
I'm totally fine with that. But if this adds any issues with the normal
case, then it's a show stopper.

> 
> > To be honest, I'm uncomfortable with this approach. It seems to be
> > fighting a symptom and not the disease. I'd rather find a way to keep
> > work from being queued on wrong CPU. If it is a timer, find a way to
> > move the timer. If it is something else, lets work to fix that. Doing
> > searches of possibly all CPUs (unlikely, but it is there), just seems
> > wrong to me.
> 
> As Vincent pointed out, on big LITTLE systems we just don't want to
> serve works on big cores. That would be wasting too much of power.
> Specially if we are going to wake up big cores.
> 
> It would be difficult to control the source driver (which queues work) to
> little cores. We thought, if somebody wanted to queue work on current
> cpu then they must use queue_work_on().

As Tejun has mentioned earlier, is there any assumptions anywhere that
expects an unbounded work queue to not migrate? Where per cpu variables
might be used. Tejun had a good idea of forcing this to migrate the work
*every* time. To not let a work queue run on the same CPU that it was
queued on. If it can survive that, then it is probably OK. Maybe add a
config option that forces this? That way, anyone can test that this
isn't an issue.

-- Steve