linux-kernel - Re: balance storm

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Message-ID: <alpine.DEB.2.02.1405272245050.21720@ionos.tec.linutronix.de>
Date:	Tue, 27 May 2014 22:53:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Libo Chen <libo.chen@...wei.com>
cc:	Mike Galbraith <umgwanakikbuti@...il.com>, mingo@...e.hu,
	LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Li Zefan <lizefan@...wei.com>, peterz@...radead.org
Subject: Re: balance storm

On Tue, 27 May 2014, Libo Chen wrote:
> On 2014/5/27 17:55, Mike Galbraith wrote:
> > On Tue, 2014-05-27 at 15:56 +0800, Libo Chen wrote: 
> >> > On 2014/5/26 22:19, Mike Galbraith wrote:
> >>> > > On Mon, 2014-05-26 at 20:16 +0800, Libo Chen wrote: 
> >>>> > >> On 2014/5/26 13:11, Mike Galbraith wrote:
> >>> > > 
> >>>>> > >>> Your synthetic test is the absolute worst case scenario.  There has to
> >>>>> > >>> be work between wakeups for select_idle_sibling() to have any chance
> >>>>> > >>> whatsoever of turning in a win.  At 0 work, it becomes 100% overhead.
> >>>> > >>
> >>>> > >> not synthetic, it is a real problem in our product. under no load, waste
> >>>> > >> much cpu time.
> >>> > > 
> >>> > > What happens in your product if you apply the commit I pointed out?
> >> > 
> >> > under no load, cpu usage is up to 60%, but the same apps cost 10% on
> >> > susp sp1.  The apps use a lot of timer.
> > Something is rotten.  3.14-rt contains that commit, I ran your test with
> > 256 threads on 64 core box, saw ~4%.
> > 
> > Putting master/nopreempt config on box and doing the same test, box is
> > chewing up truckloads of CPU, but not from migrations. 
> > 
> > perf top -g --sort=symbol
> in my box:
> 
> perf top -g --sort=symbol
> 
> Events: 3K cycles
>  73.27%  [k] read_hpet

Why is that machine using read_hpet() ?

Please provide the output of 

# dmesg | grep -i tsc

and

# cat /sys/devices/system/clocksource/clocksource0/available_clocksource

and

# cat /sys/devices/system/clocksource/clocksource0/current_clocksource

Thanks,

	tglx
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