Ingo Molnar wrote:
* Chuck Ebbert <[email protected]> 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?
a quick check on two p2/p4 boxes gives the cycle numbers below. syscall/io times for comparison.
Not that cheap, but still only ~1/4 of a syscall...
P4 P2
123 39 {movl $-47,%%eax ; movl $((27<<3)|3),%%edx ; lsll %%edx,%%eax ; jnz 1f ; andl $0xff,%%eax ; 1: ;} (average: 155)
959 287 {movl $20,%%eax ; int $0x80 ; # getpid() } (average: 983)
475 153 {movl $20,%%eax ; call *vsyscall ; # getpid() } (average: 519)
333 586 {outb %%al,$0x80;} (average: 369)
3572 1181 {outb %%al,$0x80;outb %%al,$0x80;} (average: 3628)
6755 1557 {outb %%al,$0x80;outb %%al,$0x80;outb %%al,$0x80;} (average: 6866)
P2:
cpu family : 6
model : 5
model name : Pentium II (Deschutes)
stepping : 2
cpu MHz : 400.982
cache size : 512 KB
P4:
cpu family : 15
model : 4
model name : Intel(R) Celeron(R) CPU 2.53GHz
stepping : 1
cpu MHz : 2533.270
cache size : 256 KB
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