Re: [PATCH 2/4] x86 mmiotrace: fix relay-buffer-full flag for SMP

[Eric Dumazet]

Pekka Paalanen a écrit :
> On Tue, 05 Feb 2008 21:44:07 +0100
> Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
>
> > Pekka Paalanen a écrit :
> > > diff --git a/arch/x86/kernel/mmiotrace/mmio-mod.c b/arch/x86/kernel/mmiotrace/mmio-mod.c
> > > index 82ae920..f492b65 100644
> > > --- a/arch/x86/kernel/mmiotrace/mmio-mod.c
> > > +++ b/arch/x86/kernel/mmiotrace/mmio-mod.c
> > > @@ -47,9 +48,13 @@ struct trap_reason {
> > >  	int active_traces;
> > >  };
> > >  
> > > +/* Accessed per-cpu. */
> > >  static struct trap_reason pf_reason[NR_CPUS];
> > >  static struct mm_io_header_rw cpu_trace[NR_CPUS];
> > >  
> > > +/* Access to this is not per-cpu. */
> > > +static atomic_t dropped[NR_CPUS];
> > > +
> > Please dont introduce NR_CPUS new arrays, since people are working hard to zap 
> > them from kernel.
> >
> > You probably can use a per_cpu variable ?
>
> Yes, it would probably be more appropriate to use DEFINE_PER_CPU()
> for 'pf_reason' and 'cpu_trace', but I wasn't sure since the examples
> of DEFINE_PER_CPU I saw always had integers or pointers, not
> whole structs. Is it okay for whole structs?

yes you can use a structure, you can check for example :

net/ipv4/route.c:static DEFINE_PER_CPU(struct rt_cache_stat, rt_cache_stat);

> 'dropped' on the other hand is not accessed in per-cpu style, any cpu
> may access any element. DEFINE_PER_CPU is not valid here, is it?

It is valid, you can use per_cpu() accessor to get a pointer to a particular 
cpu data.

check net/ipv4/route.c for an example :

return &per_cpu(rt_cache_stat, cpu);

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