Zou Nan hai (on 08 Feb 2007 13:11:49 +0800) wrote:
>On Thu, 2007-02-08 at 14:55, Keith Owens wrote:
>> Keith Owens (on Thu, 08 Feb 2007 17:37:54 +1100) wrote:
>> Correction: ar.k3 contains the physical address of the per-cpu data
>> area, virtual access to per-cpu data goes via the cpu local TLB and
>> does not rely on an ar.k<n> variable. ar.k3 is used in the MCA
>> assembler handler, see GET_THIS_PADDR in include/asm-ia64/mca_asm.h
>> and
>> arch/ia64/kernel/mca_asm.S.
>>
>
> Since MCA is slow path,
> so I think put smp_processor_id in ar.kr3 is a gain.
>
> We could even optimize get_cpu_var based on this...
(1) Somebody else (not me) gets to fix up and test the MCA handler
assembler code - lots of luck.
(2) smp_processor_id() in the IA64 kernel is accessed via struct
thread_info.cpu. That maps to a simple memory access with code
like this:
adds r14=3252,r13
;;
ld4 r15=[r14]
The stop bits usually get amortized away with other code.
thread_info.cpu will normally be cached in L1 so reading
smp_processor_id() is relatively fast.
(3) Reading smp_processor_id() from ar.k3 in the kernel is 10 times
slower than the existing kernel code. See the timing program
below.
(4) If the justification for storing cpu number in ar.k<n> is to speed
up user space, how can user space tell if the current kernel stores
the physical address of the per-cpu data in k3 or if it stores the
cpu number in k3? Detecting which variant of the kernel is running
will slow down user space.
Timing results on 'modprobe measure'
init_measure: empty_loop 2000007 cpu_loop 3000011 k3_loop 11999992
module measure.c
-----------------------------------------------------------------------
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/preempt.h>
#include <asm/kregs.h>
#include <asm/timex.h>
MODULE_LICENSE("GPL");
#define LOOPS 1000000
static int __init init_measure(void)
{
int loop;
register int cpu;
unsigned long start, end, empty_loop, cpu_loop, k3_loop;
printk("%s: start\n", __FUNCTION__);
preempt_disable();
local_irq_disable();
start = get_cycles();
barrier();
for (loop = 0; loop < LOOPS; ++loop) {
/* ensure that all loops are the same size (2 bundles) */
asm volatile ("nop 0; nop 0; nop 0;");
barrier();
};
end = get_cycles();
barrier();
local_irq_enable();
empty_loop = end - start;
local_irq_disable();
start = get_cycles();
barrier();
for (loop = 0; loop < LOOPS; ++loop) {
/* hand code the read of smp_processor_id() to stop gcc moving
* the address calculation outside the loop
*/
asm volatile ("adds r14=%0,r13"
";;"
"ld4 r15=[r14]"
: :
"i" (IA64_TASK_SIZE + offsetof(struct
thread_info, cpu)) :
"r14", "r15" );
barrier();
};
end = get_cycles();
barrier();
local_irq_enable();
cpu_loop = end - start;
local_irq_disable();
start = get_cycles();
barrier();
for (loop = 0; loop < LOOPS; ++loop) {
cpu = ia64_get_kr(IA64_KR_PER_CPU_DATA);
barrier();
};
end = get_cycles();
barrier();
local_irq_enable();
k3_loop = end - start;
preempt_enable();
printk("%s: empty_loop %ld cpu_loop %ld k3_loop %ld\n", __FUNCTION__,
empty_loop, cpu_loop, k3_loop);
return 0;
}
static void __exit exit_measure(void)
{
printk("%s: start\n", __FUNCTION__);
printk("%s: end\n", __FUNCTION__);
}
module_init(init_measure)
module_exit(exit_measure)
-----------------------------------------------------------------------
objdump of the interesting bits (the three loops):
empty loop:
40: 09 08 00 50 00 21 [MMI] mov r1=r40
46: 00 00 00 02 00 e0 nop.m 0x0
4c: 81 6c 64 84 adds r15=3272,r13;;
50: 0a 18 00 1e 10 10 [MMI] ld4 r3=[r15];;
56: 20 08 0c 00 42 00 adds r2=1,r3
5c: 00 00 04 00 nop.i 0x0
60: 0b 00 00 00 01 00 [MMI] nop.m 0x0;;
66: 00 10 3c 20 23 00 st4 [r15]=r2
6c: 00 00 04 00 nop.i 0x0;;
70: 0b 00 00 02 07 00 [MMI] rsm 0x4000;;
76: 50 02 b0 44 08 00 mov.m r37=ar.itc
7c: 00 00 04 00 nop.i 0x0;;
80: 0b 70 fc 78 84 24 [MMI] mov r14=999999;;
86: 00 00 00 02 00 00 nop.m 0x0
8c: e0 08 aa 00 mov.i ar.lc=r14;;
90: 01 00 00 00 01 00 [MII] nop.m 0x0
96: 00 00 00 02 00 00 nop.i 0x0
9c: 00 00 04 00 nop.i 0x0;;
a0: 10 00 00 00 01 00 [MIB] nop.m 0x0
a6: 00 00 00 02 00 a0 nop.i 0x0
ac: f0 ff ff 48 br.cloop.sptk.few 90
<init_module+0x90>
b0: 0b 20 01 58 22 04 [MMI] mov.m r36=ar.itc;;
b6: 00 00 04 0c 00 00 ssm 0x4000
bc: 00 00 04 00 nop.i 0x0;;
c0: 0b 00 00 00 30 00 [MMI] srlz.d;;
Read smp_processor_id:
c6: 00 00 04 0e 00 00 rsm 0x4000
cc: 00 00 04 00 nop.i 0x0;;
d0: 01 18 01 58 22 04 [MII] mov.m r35=ar.itc
d6: 00 00 00 02 00 00 nop.i 0x0
dc: 00 00 04 00 nop.i 0x0;;
e0: 0a 40 fc 78 84 24 [MMI] mov r8=999999;;
e6: 00 00 00 02 00 00 nop.m 0x0
ec: 80 08 aa 00 mov.i ar.lc=r8
f0: 0b 70 d0 1a 19 21 [MMI] adds r14=3252,r13;;
f6: f0 00 38 20 20 00 ld4 r15=[r14]
fc: 00 00 04 00 nop.i 0x0;;
100: 10 00 00 00 01 00 [MIB] nop.m 0x0
106: 00 00 00 02 00 a0 nop.i 0x0
10c: f0 ff ff 48 br.cloop.sptk.few f0
<init_module+0xf0>
110: 0b 10 01 58 22 04 [MMI] mov.m r34=ar.itc;;
116: 00 00 04 0c 00 00 ssm 0x4000
11c: 00 00 04 00 nop.i 0x0;;
120: 0b 00 00 00 30 00 [MMI] srlz.d;;
Read ar.k3:
126: 00 00 04 0e 00 00 rsm 0x4000
12c: 00 00 04 00 nop.i 0x0;;
130: 01 08 01 58 22 04 [MII] mov.m r33=ar.itc
136: 00 00 00 02 00 00 nop.i 0x0
13c: 00 00 04 00 nop.i 0x0;;
140: 0a 48 fc 78 84 24 [MMI] mov r9=999999;;
146: 00 00 00 02 00 00 nop.m 0x0
14c: 90 08 aa 00 mov.i ar.lc=r9
150: 01 70 00 06 22 04 [MII] mov.m r14=ar.k3
156: 00 00 00 02 00 00 nop.i 0x0
15c: 00 00 04 00 nop.i 0x0;;
160: 10 00 00 00 01 00 [MIB] nop.m 0x0
166: 00 00 00 02 00 a0 nop.i 0x0
16c: f0 ff ff 48 br.cloop.sptk.few 150
<init_module+0x150>
170: 0b 00 01 58 22 04 [MMI] mov.m r32=ar.itc;;
176: 00 00 04 0c 00 00 ssm 0x4000
17c: 00 00 04 00 nop.i 0x0;;
180: 01 00 00 00 30 00 [MII] srlz.d
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