Messages in this thread | | | Date | Fri, 1 Apr 2005 12:20:42 -0500 (EST) | From | Alan Stern <> | Subject | RE: 2.6.11, USB: High latency? |
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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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