linux-kernel - Re: [PATCH] [RT] avoid preemption in memory controller code

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Date:	Mon, 8 Dec 2008 12:23:17 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tim Blechmann <tim@...ngt.org>
cc:	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] [RT] avoid preemption in memory controller code


On Mon, 8 Dec 2008, Tim Blechmann wrote:

> > > the lru_lock of struct mem_group_per_zone is used to avoid preemption
> > > during the __mem_cgroup_stat_add_safe function.
> > > therefore, a raw_spinlock_t should be used.
> > 
> > What is the reason that this must avoid preemption? 
> 
> it guards a call to smp_processor_id() in __mem_cgroup_stat_add_safe().
> see http://article.gmane.org/gmane.linux.rt.user/3690

We could simply create a 
	preempt_disable_rt();
that will only disable preemption when CONFIG_PREEMPT_RT is set.
and then we could add

	int cpu;

	preempt_disable_rt();
	cpu = smp_processor_id();
	stat->cpustat[cpu].count[idx] += val;
	preempt_enable_rt();

Or something similar.

-- Steve

> 
> > Is there another
> > way to solve this? I rather not just add a raw spinlock if we can
> > help it.
> 
> not sure, maybe one can disable preemption for that specific function?
> 
> tim
> 
> --
> tim@...ngt.org
> http://tim.klingt.org
> 
> Art is either a complaint or do something else
>   John Cage quoting Jasper Johns
> 
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