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 13:12:41 -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:
> 
> > preempt_check_resched is basically:
> >
> > a test TIF_NEED_RESCHED
> >   if true, call to preempt_schedule
> 
> You did not mention the effect of incrementing the preempt counter and
> the barrier(). Adds an additional cacheline to a very hot OS path.
> Possibly register effects.
> 

What you say applies to preempt_enable(). I was describing
preempt_check_resched above, which involves no compiler barrier nor
increment whatsoever.

By the way, the barrier() you are talking about is in
preempt_enable_no_resched(), the very primitive you are considering
using to save these precious cycles.


> > 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.
> 
> I am trying to get rid off all unnecessary overhead. These "incredible
> cheap" tricks en masse have caused lots of regressions. And the allocator
> hotpaths are overloaded with these "incredibly cheap" checks alreayd.
> 
> > 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()
> 
> Ok. If you have to use preempt then you have to suffer I guess..
> 

Yes. A user enabling full preemption should be aware that it has a 
performance footprint.

By the way, from what I remember of the slub allocator, you might find 
the following more suited for your needs. I remember that the slow path
sometimes need to reenable interrupts, so:

preempt disable()
fast path attempt
  if (fast path already taken) {
    local_irq_save();
    preempt_enable_no_resched();
    slow path {
      if (!flags & GFP_ATOMIC) {
        local_irq_enable();
        preempt_check_resched();
        ...
        local_irq_disable();
      }
    }
    local_irq_restore();
    preempt_check_resched();
    return;
  }
preempt_enable()

This should work, be efficient and manage to ensure scheduler RT
correctness.

Mathieu

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