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

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Message-ID: <4A39B25C.2040801@cn.fujitsu.com>
Date:	Thu, 18 Jun 2009 11:19:56 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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?

	Thanks, Lai.

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