Date: Thu, 15 Jan 2009 19:39:42 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
       Brian Gerst <brgerst@gmail.com>, ebiederm@xmission.com,
       cl@linux-foundation.org, rusty@rustcorp.com.au, travis@sgi.com,
       linux-kernel@vger.kernel.org, steiner@sgi.com, hugh@veritas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
Message-ID: <20090115183942.GA6325@elte.hu>
References: <496F0F50.6070200@kernel.org> <20090115113045.GG22850@elte.hu> <496F2032.5080502@kernel.org> <20090115122611.GJ22850@elte.hu> <496F346B.6020804@kernel.org> <20090115130740.GE21839@elte.hu> <496F38C7.5000403@kernel.org> <20090115093007.4215b4b4.akpm@linux-foundation.org> <20090115180259.GJ22472@elte.hu> <20090115103406.a85968f0.akpm@linux-foundation.org>
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* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 15 Jan 2009 19:02:59 +0100 Ingo Molnar <mingo@elte.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@linux-foundation.org> wrote:
> > 
> > > On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@kernel.org> wrote:
> > > 
> > > > --- a/include/asm-generic/percpu.h
> > > > +++ b/include/asm-generic/percpu.h
> > > > @@ -80,4 +80,56 @@ extern void setup_per_cpu_areas(void);
> > > >  #define DECLARE_PER_CPU(type, name) extern PER_CPU_ATTRIBUTES \
> > > >  					__typeof__(type) per_cpu_var(name)
> > > >  
> > > > +/*
> > > > + * Optional methods for optimized non-lvalue per-cpu variable access.
> > > > + *
> > > > + * @var can be a percpu variable or a field of it and its size should
> > > > + * equal char, int or long.  percpu_read() evaluates to a lvalue and
> > > > + * all others to void.
> > > > + *
> > > > + * These operations are guaranteed to be atomic w.r.t. preemption.
> > > > + * The generic versions use plain get/put_cpu_var().  Archs are
> > > > + * encouraged to implement single-instruction alternatives which don't
> > > > + * require preemption protection.
> > > > + */
> > > > +#ifndef percpu_read
> > > > +# define percpu_read(var)						\
> > > > +  ({									\
> > > > +	typeof(per_cpu_var(var)) __tmp_var__;				\
> > > > +	__tmp_var__ = get_cpu_var(var);					\
> > > > +	put_cpu_var(var);						\
> > > > +	__tmp_var__;							\
> > > > +  })
> > > > +#endif
> > > 
> > > I wonder if the preempt_disable()/preempt_enable() in here actually
> > > does anything useful on any architecture.
> > 
> > Provides "this is IRQ safe"
> 
> ?
> 
> > and "this is preempt safe" semantics.
> 
> Of course.  But do any architectures actually _need_ that for a single 
> read?

not for a read i guess - but for the other ops like add/and/or/xor.

> Maybe.  And if so, they can interpose their arch-specific 
> implementation.  But if the generic version is optimal for them, they 
> wouldn't need to..

we cannot turn the generic ops into a single instruction so arch methods 
are preferable no matter how thick or thin the generic version is. But i 
agree that the optimization you suggest could be done.

	Ingo
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