linux-kernel - Re: PREEMPT is messing with everyone

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Date:	Tue, 05 Dec 2006 16:08:07 +0100
From:	Michal Schmidt <xschmi00@...d.feec.vutbr.cz>
To:	Jaswinder Singh <jaswinderrajput@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: PREEMPT is messing with everyone

Jaswinder Singh wrote:
> Hi,
> 
> preempt stuff SHOULD only stay in #ifdef CONFIG_PREEMP_* , but it is
> messing with everyone even though not defined.
> 
> e.g.
> 
> 1. linux-2.6.19/kernel/spinlock.c
> 
> Line 18: #include <linux/preempt.h>
> 
> Line 26:  preempt_disable();
> 
> Line 32:  preempt_disable();
> 
> and so on .

Don't worry. These compile into "do { } while (0)" (i.e. nothing) when 
CONFIG_PREEMPT is not set.

> 
> 2. linux-2.6.19/kernel/sched.c
> 
> Line 1096:  int preempted;
> 
> Line 1104:   preempted = !task_running(rq, p);
> 
> Line 1106:   if (preempted)
> 
> Line 2059:  if (TASK_PREEMPTS_CURR(p, this_rq))

Linux always does preemptive multitasking of user tasks. These have 
nothing to do with CONFIG_PREEMPT.

> Line 3355:    current->comm, preempt_count(), current->pid);
> 
> Line 3342:  preempt_disable();
> 
> Line 3375:  if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {

preempt_count() is useful in !CONFIG_PREEMPT kernels too. It stores 
information about the current context (hardirq, softirq, ...).

> [...]
> 
> 70 to 80 % of this code is removed when compiled.
> 
> but 20 to 30 % code left in binary kernel image.
> 
> Why Linux kernel is wasting its resources which is not defined at all.

I don't think that's the case.

> Any solution ?
> 
> Thank you,
> 
> Best Regards,
> 
> Jaswinder Singh.

Michal
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