Re: [PATCH RFC tip/core/rcu 3/4] rcu: Make RCU_FAST_NO_HZ account for pauses out of idle

Paul E. McKenney
2012-04-26T14:50:14Z

On Thu, Apr 26, 2012 at 03:00:42PM +0200, Peter Zijlstra wrote:
> On Mon, 2012-04-23 at 09:16 -0700, Paul E. McKenney wrote:
> > static DEFINE_PER_CPU(int, rcu_dyntick_drain);
> > static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
> > static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
> > +static DEFINE_PER_CPU(unsigned long, rcu_idle_gp_timer_expires);
> > +static DEFINE_PER_CPU(bool, rcu_idle_first_pass);
> > +static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted);
> > +static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted_snap);
>
>
> Wouldn't that all be prettier if it were in a struct of sorts?
>
> struct rcu_dyntick {
> int drain;
> unsigned long holdoff;
> struct timer_list gp_timer;
> unsigned long gp_timer_expires;
> bool first_pass;
> unsigned long posted;
> unsigned long posted_snap;
> };
>
> static DEFINE_PER_CPU(struct rcu_dyntick, rcu_dyntick);
>
> ( fwiw, bool doesn't have a well specified storage type )
>
> This way you have more control over the placement, variables are forced
> to be together, instead of at the mercy of whatever per_cpu and the
> linker do, and you more clearly see the holes in the structure.
>
> All the per_cpu() usage should still work like:
>
> per_cpu(rcu_dyntick.first_pass, cpu) = 0;

Excellent point -- I have added that transformation to the todo list for 3.6.

I must confess that I had no idea that you could say the above. I would
have expected to have to say the following:

per_cpu(rcu_dyntick, cpu).first_pass = 0;

Not much difference either way, though.

Thanx, Paul