Re: USB interrupt times

Hi Russell,

On Tue, Aug 14, 2012 at 12:54:00PM +0200, Jiri Kosina wrote:

quoted

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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