linux-kernel - Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable

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Date:	Wed, 7 Oct 2009 11:41:11 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Tejun Heo <tj@...nel.org>, Mel Gorman <mel@....ul.ie>,
	mingo@...e.hu
Subject: Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o
	interrupt disable

* Christoph Lameter (cl@...ux-foundation.org) wrote:
> On Wed, 7 Oct 2009, Mathieu Desnoyers wrote:
> 
> > You are already calling the scheduler when ending the _fast_ path. I
> > don't see the problem with calling it when you end the slow path
> > execution.
> 
> Well yes that gives rise to the thought of using
> 
> 	preempt_enable_no_sched()
> 
> at the end of the fastpath as well. Constant calls into the scheduler
> could be a major performance issue

... but the opposite is a major RT behavior killer.

preempt_check_resched is basically:

a test TIF_NEED_RESCHED
  if true, call to preempt_schedule

So, it's not a call to the scheduler at each and every preempt_enable.
It's _only_ if an interrupt came in during the preempt off section and
the scheduler considered that it should re-schedule soon.

I really don't see what's bothering you here. Testing a thread flag is
incredibly cheap. That's what is typically added to your fast path.

So, correct behavior would be:

preempt disable()
fast path attempt
  if (fast path already taken) {
    local_irq_save();
    slow path.
    local_irq_restore();
  }
preempt_enable()


Mathieu

> . I dont notice it here since I usually
> cannot affort the preempt overhead and build kernels without support for
> it.
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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