Re: [PATCH] new irq tracer

[Jason Baron <jbaron@redhat.com>]

On Wed, Feb 18, 2009 at 10:15:25PM +0100, Peter Zijlstra wrote:
> On Wed, 2009-02-18 at 14:53 -0500, Jason Baron wrote:
> > hi,
> > 
> > Using a copule of tracepoints i've implemented an 'irq tracer' which has
> > the following column format:
> > 
> > [time][irq][action][handled][cpu][length]
> > 
> > for example on my system:
> > 
> > [ 1038.927248381]     28     eth0      1        3     0.000002745
> > [ 1038.927544688]     28     eth0      1        3     0.000002520
> > [ 1038.927593609]     28     eth0      1        3     0.000002509
> > [ 1038.974087675]     29     ahci      1        1     0.000013135
> > [ 1038.974288475]     29     ahci      1        1     0.000005542
> > [ 1038.974414324]     29     ahci      1        1     0.000007953
> > 
> > I think this patch is useful in understanding irq system behavior, and
> > for tracking down irq driver handlers that are firing too often or
> > spending too much time handling an irq. This approach could be extended
> > to the timer irq and for softirqs...
> 
> The function graph tracer can already do this, it has a special section
> to recognize irq entry function, and already measures time spend.
> 
> I guess with Steve's new triggers (probes or whatever they ended up
> being called) you could limit it to just IRQ entry points.
> 

i did look at the graph tracer first. While it does measure irq related
functions, it does not give this level of detail concerning which irq #,
which irq handler is involved, and whether or not the irq was handled
successfully or not. So I believe this tracepoints add a level of detail
that the graph tracer does not have. Furthermore, this patch requires 2
tracepoints, not instrumentation for all kernel functions.

thanks,

-Jason