Messages in this thread |  | | Subject | Re: Feedback on preemptible kernel patch | From | Robert Love <> | Date | 14 Sep 2001 11:01:41 -0400 |
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On Fri, 2001-09-14 at 03:30, george anzinger wrote: > Right, the same problem as using floating point in the kernel (mmx uses > the FP regs and they are not saved).
Right, and I suspect we will find more problems of this type as we go on. In fact, the more general case "things that are SMP-safe but not preempt safe" will be issues, too. The highmem bug was one of these - code that was SMP-safe but did not have lock points because it was per-CPU code. Preemption ruins all that.
> The question is: Just how long do these routines take? If it is very long > it may be best to just say no. One way would be to always pretend that > the"in_interrupt" flag is set. I think possibly some routines are > short and the switch off/ switch on pair is right, but for the long ones, > well the preemption patch is supposed to make the kernel more preemptable, > not less. Any one have execution times for these functions?
Well, its the routines in arch/i386/lib/mmx.c -- and just the ones that call kernel_begin/end_fpu. My patch pushes a ctx_sw_off/on pair into those functions. Anyhow, if you look, they aren't too long.
However, I agree that we may be destroying our purpose here. A user of the patch actually put together a patch that will disable the CONFIG to use the fast MMX memcpy stuff if preemption was enabled. He benchmarked against the two and I can send you those results when I sort through them.
-- Robert M. Love rml at ufl.edu rml at tech9.net
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