Re: [PATCH] Instrumenting high latency

[Andrew Morton <akpm@osdl.org>]

Con Kolivas <kernel@kolivas.org> 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
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.