On Fri, 1 Apr 2005, kus Kusche Klaus wrote:
> > The biggest advantage would come from using a bottom-half
> > handler to do
> > most of the work. Right now the uhci-hcd driver does
> > everything in its
> > interrupt handler. This would certainly help IRQ latency; it
> > might not
> > affect application latency very much.
>
> Sounds very reasonable, thanks. Also helps application latency,
> because with the RT patches, I can tune the rt prio of softirq
> execution (that's where bottom-half goes, doesn't it?) w.r.t. the
> rt prio of the application threads.
Yes. Bear in mind, however, that if these application threads are doing
I/O over USB then they will be forced to wait for the bottom half to
execute, regardless of its priority.
> However, if I understand things correctly, if you really need
> to disable all interrupts while doing the USB work, it will not
> make any difference if IRQs are disabled while you are in the
> USB IRQ handler, or if they are disabled for the same amount of
> work/time in the bottom-half code.
For most of the USB work it will be necessary only to insure that no more
than one copy of the bottom-half handler is running at a time, which I
think the kernel does automatically for tasklets. There are a few places
where all IRQs will have to be disabled, but those places are relatively
small and short.
Right now, of course, everything runs with IRQs disabled.
> > We'll see what happens with the upcoming changes. Maybe
> > you'll be able to
> > test them for me?
>
> Basically, yes (as long as our company doesn't decide to stop the
> linux experiments).
>
> However, I depend on Ingo's RT patch, which is against the -rc series,
> not against the -mm series. So I will probably not be able to apply
> patches created against -mm.
Okay. It will be a while before the new code is ready and the changes on
which it depends have gotten into -rc.
Alan Stern
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