Messages in this thread Patch in this message | | | From | Marin Mitov <> | Subject | [PATCH]new_TSC_based_delay_tsc() | Date | Tue, 20 Nov 2007 21:32:27 +0200 |
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Hi all,
Please ignore the previous patch with the same subject. It has a bug that can manifest itself in the very exotic case when each do {} while() iteration executes on different cpu leading to potentially infinite loop.
This is a patch based on the Ingo's idea/patch to track delay_tsc() migration to another cpu by comparing smp_processor_id(). It is against kernel-2.6.24-rc3.
What is different: 1. Using unsigned (instead of long) to unify for i386/x86_64. 2. Minimal preempt_disable/enable() critical sections (more room for preemption) 3. some statements have been rearranged, to account for possible under/overflow of left/TSC
Tested on both: 32/64 bit SMP PREEMPT kernel-2.6.24-rc3
Comments, please.
Signed-off-by: Marin Mitov <mitov@issp.bas.bg> ========================================= --- a/arch/x86/lib/delay_32.c 2007-11-18 08:14:05.000000000 +0200 +++ b/arch/x86/lib/delay_32.c 2007-11-20 19:03:52.000000000 +0200 @@ -38,18 +38,42 @@ :"0" (loops)); } -/* TSC based delay: */ +/* TSC based delay: + * + * We are careful about preemption as TSC's are per-CPU. + */ static void delay_tsc(unsigned long loops) { - unsigned long bclock, now; + unsigned prev, prev_1, now; + unsigned left = loops; + unsigned prev_cpu, cpu; + + preempt_disable(); + rdtscl(prev); + prev_cpu = smp_processor_id(); + preempt_enable(); + now = prev; - preempt_disable(); /* TSC's are per-cpu */ - rdtscl(bclock); do { rep_nop(); + + left -= now - prev; + prev = now; + + preempt_disable(); + rdtscl(prev_1); + cpu = smp_processor_id(); rdtscl(now); - } while ((now-bclock) < loops); - preempt_enable(); + preempt_enable(); + + if (prev_cpu != cpu){ + /* + * We have migrated, forget prev_cpu's tsc reading + */ + prev = prev_1; + prev_cpu = cpu; + } + } while ((now-prev) < left); } /* --- a/arch/x86/lib/delay_64.c 2007-11-18 08:14:40.000000000 +0200 +++ b/arch/x86/lib/delay_64.c 2007-11-20 19:47:29.000000000 +0200 @@ -26,18 +26,42 @@ return 0; } +/* TSC based delay: + * + * We are careful about preemption as TSC's are per-CPU. + */ void __delay(unsigned long loops) { - unsigned bclock, now; + unsigned prev, prev_1, now; + unsigned left = loops; + unsigned prev_cpu, cpu; + + preempt_disable(); + rdtscl(prev); + prev_cpu = smp_processor_id(); + preempt_enable(); + now = prev; - preempt_disable(); /* TSC's are pre-cpu */ - rdtscl(bclock); do { - rep_nop(); + rep_nop(); + + left -= now - prev; + prev = now; + + preempt_disable(); + rdtscl(prev_1); + cpu = smp_processor_id(); rdtscl(now); - } - while ((now-bclock) < loops); - preempt_enable(); + preempt_enable(); + + if (prev_cpu != cpu){ + /* + * We have migrated, forget prev_cpu's tsc reading + */ + prev = prev_1; + prev_cpu = cpu; + } + } while ((now-prev) < left); } EXPORT_SYMBOL(__delay); - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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