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SubjectRe: [RFC, patch] i386: vgetcpu(), take 2
In-Reply-To: <20060621081539.GA14227@elte.hu>

On Wed, 21 Jun 2006 10:15:39 +0200, Ingo Molnar wrote:

> * Chuck Ebbert <76306.1226@compuserve.com> wrote:
>
> > Use a GDT entry's limit field to store per-cpu data for fast access
> > from userspace, and provide a vsyscall to access the current CPU
> > number stored there.
>
> very nice idea! I thought of doing sys_get_cpu() too, but my idea was to
> use the scheduler to keep a writable [and permanently pinned,
> per-thread] VDSO data page uptodate with the current CPU# [and other
> interesting data]. Btw., do we know how fast LSL is on modern CPUs?

Now that the GDT is a full page for each CPU there's plenty of space
for all kinds of per-cpu data, even if we waste 75% of it. LSL seems
pretty fast; I got 13 clocks for the whole lsl/jnz/and sequence on K8
and 21 clocks on PII. Myabe you can test P4?

/* test how fast lsl/jnz/and runs.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#define rdtscll(t) asm volatile ("rdtsc" : "=A" (t))

#ifndef ITERS
#define ITERS 1000000
#endif
int main(int argc, char * const argv[])
{
unsigned long long tsc1, tsc2;
int count, cpu, junk;
rdtscll(tsc1);
asm (
" pushl %%ds \n"
" popl %2 \n"
"1: \n"
#ifdef DO_TEST
" lsl %2,%0 \n"
" jnz 2f \n"
" and $0xff,%0 \n"
#endif
" dec %1 \n"
" jnz 1b \n"
"2: \n"
: "=&r" (cpu), "=&r" (count), "=&r" (junk)
: "1" (ITERS), "0" (-1)
);
rdtscll(tsc2);

if (count == 0)
printf("loops: %d, avg: %llu clocks\n",
ITERS, (tsc2 - tsc1) / ITERS);
return 0;
}

> > +__vgetcpu:
> > +.LSTART_vgetcpu:
> > + movl $-EFAULT,%eax
> > + movl $((27<<3)|3),%edx
> > + lsll %edx,%eax
> > + jnz 1f
> > + andl $0xff,%eax
> > +1:
> > + ret
>
> this needs unwinder annotations as well to make this a proper DSO, so
> that for example a breakpoint here does not confuse gdb.

I can't write those.

> also, would be nice to do something like this in 64-bit mode too.

Andi has x86_64 patches in his tree and is considering this method for
ia32 support.

--
Chuck
"You can't read a newspaper if you can't read." --George W. Bush
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