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Date: Sat, 20 Jun 2009 17:48:57 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Zhaolei <zhaolei@cn.fujitsu.com>, Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       fweisbec@gmail.com, Li Zefan <lizf@cn.fujitsu.com>,
       Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH RFC] softirq: fix ksoftirq starved
Message-ID: <20090620154857.GB12901@elte.hu>
References: <4A0BF82A.2070208@cn.fujitsu.com> <20090514124013.GC21241@Krystal> <alpine.DEB.2.00.0905140926010.30591@gandalf.stny.rr.com> <4A0CCB2E.10202@cn.fujitsu.com> <6A70E08D65F749FA92475F8BA1F9510D@zhaoleiwin> <20090519082435.GB15286@elte.hu> <4A31BF1D.1050400@cn.fujitsu.com> <4A32251C.8060001@cn.fujitsu.com> <20090617145333.GD6846@elte.hu> <4A39B25C.2040801@cn.fujitsu.com>
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* Lai Jiangshan <laijs@cn.fujitsu.com> wrote:

> Ingo Molnar wrote:
> > * Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
> > 
> >> --- a/kernel/sched.c
> >> +++ b/kernel/sched.c
> >> @@ -5307,6 +5307,7 @@ need_resched:
> >>  	release_kernel_lock(prev);
> >>  need_resched_nonpreemptible:
> >>  
> >> +	schedule_softirq_check();
> >>  	schedule_debug(prev);
> > 
> > hm, this slows down the scheduler fast-path ...
> > 
> > 	Ingo
> > 
> > 
> 
> It's true. But:
> 
> The overheads are:
> 
> Overhead-A: the function call statement "schedule_softirq_check();"
> It can be gotten rid off by a macro or inline function.
> 
> Overhead-B: __get_cpu_var() and the test statement.
> 
> Overhead-C: do_softirq()
> In my patch, we test a variable and then call do_softirq() when
> the variable is true. do_softirq() can be called from process
> context or from schedule() or by any other ways, but it must be
> called and avoids starvation in this condition.
> So we need pay this overhead. It is no worse than before.
> 
> Is it a critical thing when it slows down the scheduler fast-path
> because of the "Overhead-B"?
> 
> Or I misunderstand something?

The thing is that _any_ extra instruction in the scheduler fast-path 
should be avoided. I dont think it can be claimed that this problem 
can only be solved that way.

	Ingo