linux-kernel - Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu

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Date:	Tue, 20 Sep 2011 12:03:28 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...two.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu_read()

On Tue, 2011-09-20 at 10:59 -0500, Christoph Lameter wrote:
> On Tue, 20 Sep 2011, Steven Rostedt wrote:
> 
> > On Tue, 2011-09-20 at 09:51 -0500, Christoph Lameter wrote:
> >
> > > I see that __this_cpu_xx operations may not work as intended in
> > > preemptable contexts and there we could have more changes.
> >
> > Then why do you use it in slub.c?
> >
> > in mm/slab.c slab_alloc():
> >
> > redo:
> >
> > 	/*
> > 	 * Must read kmem_cache cpu data via this cpu ptr. Preemption is
> > 	 * enabled. We may switch back and forth between cpus while
> > 	 * reading from one cpu area. That does not matter as long
> > 	 * as we end up on the original cpu again when doing the cmpxchg.
> > 	 */
> > 	c = __this_cpu_ptr(s->cpu_slab);
> >
> > The __this_cpu_*() is in preempt enabled location. In fact, the three
> > this_cpu_write()'s in acquire_slab() is done within a spinlock and thus
> > with preemption disabled.
> 
> So the this_cpu_writes could become __this_cpu_writes
> 
> The __this_cpu_ptr operation in slab_alloc is special. We explicitly do
> address calculation and do not care from which per cpu area we fetch
> multiple per cpu data items because we can verify that we are on the same
> per cpu area during the this_cpu_cmpxchg_double operation.

ARGH! You now have special cases that break the sanity of it all? This
just proves that the design is wrong.

> 
> > The point is, people get it wrong all the time. In fact, we should
> > really require that ALL USES of this_cpu_*() must be with preemption
> > disabled. Regardless. Because anytime you touch a per cpu variable,
> 
> NO!! This defeats the whole purpose of this_cpu_ops and make the whole
> scheme utterly useless.

The thing is, the whole purpose was broken to begin with. Defeating a
broken design is a good thing!

> 
> > Monkeying around with per cpu data is tricky. If you start doing it in
> > preempt enabled code, you are most certainly about to get it wrong. Why
> > have this super optimization. A preempt_disable() is a single operation
> > that touches cache hot data.
> 
> There are trivial cases like counter increments that are not a problem at
> all. Most use cases are those. More complex ones can be developed to avoid
> various overhead in performance critical sections of the kernel.
> 

And adding a preempt_disable; this_cpu_inc(); preempt_enable; is not a
bad thing either.

What benchmarks do you have that shows this helped in anything????

-- Steve


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