linux-kernel - Re: [PATCH RFC] softirq: fix ksoftirq starved

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Date:	Sat, 20 Jun 2009 17:48:57 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Zhaolei <zhaolei@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	Li Zefan <lizf@...fujitsu.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC] softirq: fix ksoftirq starved


* Lai Jiangshan <laijs@...fujitsu.com> wrote:

> Ingo Molnar wrote:
> > * Lai Jiangshan <laijs@...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
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