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 - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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