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

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Message-ID: <20091008172221.GB3370@Krystal>
Date:	Thu, 8 Oct 2009 13:22:21 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...ux-foundation.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

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Thu, 2009-10-08 at 08:44 -0400, Mathieu Desnoyers wrote:
> > Even if only done with interrupt off, and check resched is called after
> > each irq enable following this critical section ? I'd like to understand
> > the reason behind your rejection for this specific case.
> 
> No, the thing you proposed:
> 
> > 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()
> 
> Seems ok.
> 
> I just don't get why Christoph is getting all upset about the
> need_resched() check in preempt_enable(), its still cheaper than poking
> at the interrupt flags.

I agree with you. need_resched() check is incredibly cheap. And if
Christoph still complains about the compiler barrier in preempt
enable_no_resched/disable, then I think he should consider the fact that
the compiler does not perform cross-function optimizations, and consider
putting the preempt disable/enable statements close to function
boundaries. Therefore, the impact in terms of compiler optimization
restrictions should be minimal.

The scheme I proposed above should be OK in terms of scheduler effect
and permit to deal with re-enabling preemption in the slow path
appropriately.

Mathieu


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