Re: [PATCH RFC] softirq: fix ksoftirq starved

From: Lai Jiangshan
Date: Wed Jun 17 2009 - 23:18:20 EST

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