Re: [PATCH] Instrumenting high latency

[Andrew Morton]

Con Kolivas <kernel@xxxxxxxxxxx> wrote:
>
> He hacked 
>  together this simple patch which times periods according to the preempt 
>  count.

OK, small problem.  We have code which does, effectively,

	if (need_resched()) {
		drop_the_lock();
		schedule();
		grab_the_lock();
	}

so if need_resched() stays false then this will hold the lock for a long
time and bogus reports are generated:

46ms non-preemptible critical section violated 1 ms preempt threshold starting at exit_mmap+0x26/0x188 and ending at exit_mmap+0x154/0x188

To fix that you need to generate high scheduling pressure so that
need_resched() is frequently true.  On all CPUs.  Modify realfeel to pin
itself to each CPU, or something like that.

This rather decreases the patch's usefulness.

The way I normally do this stuff is with

	http://www.zip.com.au/~akpm/linux/patches/stuff/rtc-debug.patch

and `amlat', from http://www.zip.com.au/~akpm/linux/amlat.tar.gz


It _might_ be sufficient to redefine need_resched() to just return 1 all
the time.  If that causes the kernel to livelock then we need to fix that
up anyway.
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