Re: USB interrupt times

["Henrik Rydberg" <rydberg-Hk7bIW8heu4wFerOooGFRg@public.gmane.org>]

Hi Russell,

> On Tue, Aug 14, 2012 at 12:54:00PM +0200, Jiri Kosina wrote:
> > Actually, Henrik (added to CC) has been doing some latency improvements 
> > both for input core in general, and for HID devices as well lately. I 
> > still have his patchset in my to-review queue, as I have just came back 
> > from offline vacation, but the patch below definitely can't hurt and 
> > should significantly lower the time spent in handling the irq for hid 
> > device in common situation (i.e. noone listening for debugfs events).
> > 
> > Could you please measure how much it helps on your system?
> 
> Ok, it looks like it's changed the maximum USB interrupt execution
> time from around 364us to 255us.

With the on-review input patches on top of that, the latency should
drop further. I have measured a total of 2.5 times lower latency in
other areas, so I would expect something like 150 us in 3.7 for your
case.

> If I also do a similar trick with the debug code in hid_input_report()
> then I get down to 212us - iow, something like the patch below.

Linus' master already has a patch for that code path, actually.

> Given that debugfs is fairly ubiquitous in the kernel, and that this is
> fairly invasive in terms of interrupt execution impact, wouldn't having
> this debug code enabled by a separate Kconfig symbol be reasonable too?

I would like that, too.

Thanks,
Henrik
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