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SubjectRe: Calling current() from interrupt context
XXXXXXXX
"Jeff V. Merkey" wrote:
>
> "Jeff V. Merkey" wrote:
> >
> > "David S. Miller" wrote:
> > >
> > > Date: Tue, 10 Oct 2000 00:44:58 +0200
> > > From: "Andi Kleen" <ak@suse.de>
> > >
> > > On Mon, Oct 09, 2000 at 11:41:13PM +0100, Alan Cox wrote:
> > > > I dont actually know a CPU that doesnt have one in SMP mode. Its just often
> > > > behind an I/O interface that is slower than ideal
> > >
> > > Where would you put it for IA32 ? I don't know any free MSR that could
> > > be abused for that, and acessing MSRs is insanely slow anyways. Any
> > > other ideas ?
> > >
> > > The local APIC holds the hardware cpu number (which happens to equal
> > > the logical cpu number in the current x86 code). And I believe the
> > > local APIC has a 32-bit scratch register or 2 as well... but don't
> > > quote me on that one.
> >
> > Accessing the CPUID from the local APIC is slower than mollasses. The
> > reason for this is due to the fact that the APIC address ranges are
> > treated as non-cacheable memory, and will always generate a
> > non-cacheable memory reference when you read from the APIC_ID register
> > and shift the ID.
>
> You can verify this with a logic analyzer and watch the system suck wind
> and get really slow, I/O performance go to shit after you read the
> memory mapped APIC registers from a context
> switch loop, i.e.
>
> while (1)
> {
> read_local_apic(); // OINK OINK OINK -- bus utilitization will go
> through the roof.
> schedule()
> }
>
> and watch it oink...
>
> >
> > A simpler solution is to use the CR2 register to store the CPU number,
> > and always reload the register after a page fault (since CR2 will hold a
> > faulting address). THis is even better than Linus' stack based method
> > for storing the number since you just are reading a register, which is
> > fast as hell. The other methods are slower.
> >
> > hHis is how it's done in NetWare and MANOS. It works....
> >
> > :-)
> >
> > Jeff
> >
> > >
> > > Later,
> > > David S. Miller
> > > davem@redhat.com
> > > -
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