Re: [PATCH 0/7] usb: host: add support for threaded IRQs

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On Thu, Apr 21, 2016 at 01:56:58PM +0300, Felipe Balbi wrote:
> 
> Hi,
> 
> Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> writes:
> > Hi,
> >
> > Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> >> On Mon, Apr 11, 2016 at 03:44:09PM +0300, Felipe Balbi wrote:
> >>> Hi guys,
> >>> 
> >>> this patchset introduces support for threaded IRQs
> >>> for host controllers drivers to use. Right now, only
> >>> XHCI has been converted, but more drivers could
> >>> easily be converted as well.
> >>> 
> >>> With this series we can, eventually, spend much less
> >>> time with IRQs disabled. Note that, because we're
> >>> masking XHCI's IRQs, we could run our thread
> >>> handlers with IRQs enabled, but I haven't tested
> >>> that yet.
> >>
> >> Does it really benifit anything?  Do you have any measurements?  Why the
> >
> > I have measurements showing that it doesn't have a negative impact.
> >
> >> added work for no real need, and increased latency?
> >
> > the latency is negligible. XHCI's irqs are used for completions and
> > aborts, not to start transfers. Also, it's a generally a good idea to
> > spend less time in hardirq context. Not to mention that this is a good
> > stepping stone for further optimization; we could (and probably should)
> > have a per-device-slot irq thread and also per-device-slot locks and
> > demote the controller's lock to only the parts which actually need to
> > access controller-wide resources (for the most part we have
> > slot-specific registers, anyway).
> >
> > Now, the benefit of the work yet to come is that we could be processing
> > N slots concurrently given we have N cpu cores.
> >
> > Note, also, that we're not forcing anybody to use this but given that
> > XHCI has these separate units referred to as 'device slots', we might as
> > well exploit that, right ?
> 
> Any further comments here ? Greg ? Mathias ?

I'm traveling this week, will get to them next week, sorry...

greg k-h
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