From mboxrd@z Thu Jan 1 00:00:00 1970 From: peterz@infradead.org (Peter Zijlstra) Date: Fri, 10 Dec 2010 20:25:56 +0100 Subject: [BUG] 2.6.37-rc3 massive interactivity regression on ARM In-Reply-To: <1292004654.13513.38.camel@laptop> References: <1291917330.6803.7.camel@twins> <1291920939.6803.38.camel@twins> <1291936593.13513.3.camel@laptop> <1291975704.6803.59.camel@twins> <1291987065.6803.151.camel@twins> <1291987635.6803.161.camel@twins> <1291988866.6803.171.camel@twins> <20101210175645.GB28263@n2100.arm.linux.org.uk> <1292004654.13513.38.camel@laptop> Message-ID: <1292009156.13513.47.camel@laptop> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, 2010-12-10 at 19:10 +0100, Peter Zijlstra wrote: > > > + delta = sched_clock_cpu(cpu_of(rq)) - rq->clock; > > > + rq->clock += delta; > > > > Hmm. Can you tell me how this is different to: > > > > new_clock = sched_clock_cpu(cpu_of(rq)); > > delta = new_clock - rq->clock; > > rq->clock = new_clock; > > > > which I think may be simpler in terms of 64-bit math for 32-bit compilers > > to deal with? > > Its not, I could write it like that, the only reason I didn't is because > it uses an extra variable. If gcc on 32bit targets really generates > hideous code for it I'll happily change it. Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -1863,6 +1863,7 @@ void account_system_vtime(struct task_st { unsigned long flags; s64 delta; + u64 now; int cpu; if (!sched_clock_irqtime) @@ -1870,8 +1871,9 @@ void account_system_vtime(struct task_st local_irq_save(flags); cpu = smp_processor_id(); - delta = sched_clock_cpu(cpu) - per_cpu(irq_start_time, cpu); - per_cpu(irq_start_time, cpu) += delta; + now = sched_clock_cpu(cpu); + delta = now - per_cpu(irq_start_time, cpu); + per_cpu(irq_start_time, cpu) = now; irq_time_write_begin(cpu); On i386 (gcc (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5): Before: account_system_vtime: 160 bytes After: account_system_vtime: 214 bytes