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SubjectRe: tbench regression with 2.6.33-rc1
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On Tue, 2010-01-12 at 00:16 +0800, Mike Galbraith wrote:
> On Mon, 2010-01-11 at 13:08 +0100, Peter Zijlstra 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, I thought to see the same on a dual-socket machine, but when
> > bisecting I ended up on a user-space perf commit, which is pretty much
> > impossible.
> >
> > I did notice some variance in the numbers between boots, maybe it was
> > large enough to fool me.. (~2800 MB/s was the good one, ~2200 MB/s was
> > the bad one).
> >
> > perf itself also didn't really provide clue, perf record -ag on the
> > workload didn't really show anything scheduler related. vmstat 1 did
> > show a proportional drop in context switch rate between the kernels
> > though.. most odd.
>
> I've been all through it too, same result. The below may make a bit of
> difference, but really has diddly spit to do with this oddity.

I test this patch applied to 2.6.33-rc3, but no help on tbench
regression.

Lin Ming

>
> netperf TCP_RR
> tip 93445 RR/sec
> tip+ 99454 RR/sec
> 1.064
>
> tbench 8
> tip 1144 MB/sec
> tip+ 1166 MB/sec
> 1.019
>
> sched: don't call wake_affine() when the result doesn't matter.
>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> LKML-Reference: <new-submission>
>
> kernel/sched_fair.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -1530,6 +1530,7 @@ static int select_task_rq_fair(struct ta
> sd = tmp;
> }
>
> +#ifdef CONFIG_GROUP_SCHED
> if (sched_feat(LB_SHARES_UPDATE)) {
> /*
> * Pick the largest domain to update shares over
> @@ -1543,9 +1544,16 @@ static int select_task_rq_fair(struct ta
> if (tmp)
> update_shares(tmp);
> }
> +#endif
>
> - if (affine_sd && wake_affine(affine_sd, p, sync))
> - return cpu;
> + if (affine_sd) {
> + if (cpu == prev_cpu)
> + return cpu;
> + if (wake_affine(affine_sd, p, sync))
> + return cpu;
> + if (!(affine_sd->flags & SD_BALANCE_WAKE))
> + return prev_cpu;
> + }
>
> while (sd) {
> int load_idx = sd->forkexec_idx;
>
>



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