Re: Instrumenting high latency

[Con Kolivas]

Andrew Morton writes:

> Con Kolivas <kernel@xxxxxxxxxxx> wrote:
> > Because of the recent discussion about latency in the kernel I asked William 
> > Lee Irwin III to help create some instrumentation to determine where in the 
> > kernel there were still sustained periods of non-preemptible code. He hacked 
> > together this simple patch which times periods according to the preempt 
> > count.
>
> Looks sane.
>
> > The patch appears to require CONFIG_PREEMPT enabled on uniprocessor and is 
> > i386 only at the moment.
>
> Not sure what you mean by "on uniprocessor"?  AFAICT the patch will work
> as-is on uniprocessor and on SMP.  Looks like it'll work with
> CONFIG_PREEMPT=n, too, although that would be a slightly bizarre thing to

That was WLI's aim, but I had one user report an OOPS with that combination.

> do.
>
> +				print_symbol("%s\n",
> +					__get_cpu_var(preempt_exit));
>
> I'll change this to
>
> 				print_symbol("%s",
> 					__get_cpu_var(preempt_exit));
> 				printk("\n");
>
> so it doesn't make a mess with CONFIG_KALLSYMS=n.

Ok 

It's not heavily tested but ran for a day on my box at home. It can trip 
over itself saying that it's own code paths are a problem too if it gets 
real busy.

Cheers,
Con

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