Re: [PATCH] percpu: add optimized generic percpu accessors

From: Ingo Molnar
Date: Thu Jan 15 2009 - 13:40:31 EST



* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, 15 Jan 2009 19:02:59 +0100 Ingo Molnar <mingo@xxxxxxx> wrote:
>
> >
> > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > On Thu, 15 Jan 2009 22:23:19 +0900 Tejun Heo <tj@xxxxxxxxxx> 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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