Messages in this thread | | | Subject | Re: tbench regression with 2.6.33-rc1 | From | Lin Ming <> | Date | Tue, 29 Dec 2009 10:09:42 +0800 |
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On Sun, 2009-12-27 at 16:32 +0800, Mike Galbraith wrote: > On Fri, 2009-12-25 at 19:11 +0800, Lin Ming wrote: > > Hi, > > > > Test machine: 16 cpus (4P/2Core/HT), 8G mem > > tbench test command: > > tbench_srv & > > tbench 32 > > > > Compared with 2.6.32, tbench has ~4% regression in 2.6.33-rc1. > > > > >>From vmstat data, the context switch number also drop ~4%. > > perf top data does not show much differences. > > > > But lockstat data shows huge difference in rq->lock, as below. > > See the attachment for the full lockstat data. > > > > Any clue of this regression? > > Nope, but I see regression I haven't been able to identify as well.
Hi, Mike
More info here, I only saw regression on Tulsa machine. No regression on Nehalem core 2 machine.
And also no regression if I run tbench with RT scheduler on tulsa machine, as below command,
schedtool -R -p 20 -e tbench_srv & schedtool -R -p 20 -e tbench 32
So seems this regression is CFS scheduler related. I also did a lot of bisect, but the result was not stable. Looks like this regression was not caused by a single commit.
Lin Ming
> > Tbench state of the union according to my box below. > > With a max performance config, 32.2 is down ~4% vs 31.9, most of which > is doing affinity checks on every wakeup. That has it's benefits too, > but definitely ain't free, or even particularly cheap for fast movers. > Would be nice to find a way to keep the ramp-up improvements without > this much cost. > > Perf also isn't free, and in 33, we lost the ability to config it out, > but even doing so via axe, there's an unidentified regression. > > As usual, bisection of tbench variance was a wasted effort. > > 2.6.31.9 = -CONFIG_PERF_COUNTERS > 2.6.31.9p = +CONFIG_PERF_COUNTERS > 2.6.32.2 = -CONFIG_PERF_EVENTS > 2.6.32.2p = +CONFIG_PERF_EVENTS > git = v2.6.33-rc2 > tip = v2.6.33-rc1-306-gae7a88c > > tbench 8 > 2.6.31.9 1190 MB/sec > 2.6.31.9p 1172 MB/sec .984 > 2.6.32.2 1146 MB/sec .977 vs 31.9 .963 > 2.6.32.2p 1129 MB/sec .985 vs 31.9 .948 > 2.6.33.git 1117 MB/sec .989 vs 31.9 .938 > 2.6.33.tip 1121 MB/sec 1.003 vs 31.9 .942 > > 2.6.32.2 1146 MB/sec 1.000 > 2.6.32.2x 1181 MB/sec 1.030 vs 31.9 .992 > > 2.6.33.git 1117 MB/sec 1.000 > 2.6.33.gitx 1145 MB/sec 1.025 vs 32.2x .969 vs 31.9 .962 (perf/hwbp off via axe) > > 2.6.32.2x diff > diff --git a/kernel/sched.c b/kernel/sched.c > index d079a9f..b71cb91 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -2363,6 +2363,9 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state, > orig_cpu = cpu; > > #ifdef CONFIG_SMP > + if (cpu == this_cpu) > + goto out_stats; > + > if (unlikely(task_running(rq, p))) > goto out_activate; > > @@ -2389,6 +2392,7 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state, > WARN_ON(p->state != TASK_WAKING); > cpu = task_cpu(p); > > +out_stats: > #ifdef CONFIG_SCHEDSTATS > schedstat_inc(rq, ttwu_count); > if (cpu == this_cpu) > >
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