Re: [PATCH RFC] select_idle_sibling experiments

[Matt Fleming <matt@codeblueprint.co.uk>]

On Tue, 05 Apr, at 02:08:22PM, Chris Mason wrote:
> 
> I started with a small-ish program to benchmark wakeup latencies.  The
> basic idea is a bunch of worker threads who sit around and burn CPU.
> Every once and a while they send a message to a message thread.
 
This reminds me of something I've been looking at recently; a similar
workload in Mel's mmtests based on pgbench with 1-client that also has
this problem of cpu_idle() being false at an inconvenient time in
select_idle_sibling(), so we move the task off the cpu and the cpu
then immediately goes idle.

This leads to tasks bouncing around the socket as we search for idle
cpus.

> It has knobs for cpu think time, and for how long the messenger thread
> waits before replying.  Here's how I'm running it with my patch:
 
[...]

Cool, I'll go have a play with this.

> Now, on to the patch.  I pushed some code around and narrowed the
> problem down to select_idle_sibling()   We have cores going into and out
> of idle fast enough that even this cut our latencies in half:
> 
> static int select_idle_sibling(struct task_struct *p, int target)
>                                 goto next;
>  
>                         for_each_cpu(i, sched_group_cpus(sg)) {
> -                               if (i == target || !idle_cpu(i))
> +                               if (!idle_cpu(i))
>                                         goto next;
>                         }
>  
> IOW, by the time we get down to for_each_cpu(), the idle_cpu() check
> done at the top of the function is no longer valid.
 
Yeah. The problem is that because we're racing with the cpu going in
and out of idle, and since you're exploiting that race condition, this
is highly tuned to your specific workload.

Which is a roundabout way of saying, this is probably going to
negatively impact other workloads.

> I tried a few variations on select_idle_sibling() that preserved the
> underlying goal of returning idle cores before idle SMT threads.  They
> were all horrible in different ways, and none of them were fast.
 
I toyed with ignoring cpu_idle() in select_idle_sibling() for my
workload. That actually was faster ;)

> The patch below just makes select_idle_sibling pick the first idle
> thread it can find.  When I ran it through production workloads here, it
> was faster than the patch we've been carrying around for the last few
> years.

It would be really nice if we had a lightweight way to gauge the
"idleness" of a cpu, and whether we expect it to be idle again soon.

Failing that, could we just force the task onto 'target' when it makes
sense and skip the idle search (and the race) altogether?