Re: [PATCH 2/2] block: Convert BDI proportion calculations to flexible proportions

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2012-06-01 at 00:42 +0200, Jan Kara wrote:
> On Fri 01-06-12 00:26:05, Peter Zijlstra wrote:
> > On Fri, 2012-06-01 at 00:11 +0200, Jan Kara wrote:
> > >  bool fprop_new_period(struct fprop_global *p, int periods)
> > >  {
> > > -       u64 events = percpu_counter_sum(&p->events);
> > > +       u64 events;
> > > +       unsigned long flags;
> > >  
> > > +       local_irq_save(flags);
> > > +       events = percpu_counter_sum(&p->events);
> > > +       local_irq_restore(flags);
> > >         /*
> > >          * Don't do anything if there are no events.
> > >          */
> > > @@ -73,7 +77,9 @@ bool fprop_new_period(struct fprop_global *p, int periods)
> > >         if (periods < 64)
> > >                 events -= events >> periods;
> > >         /* Use addition to avoid losing events happening between sum and set */
> > > +       local_irq_save(flags);
> > >         percpu_counter_add(&p->events, -events);
> > > +       local_irq_restore(flags);
> > >         p->period += periods;
> > >         write_seqcount_end(&p->sequence); 
> > 
> > Uhm, why bother enabling it in between? Just wrap the whole function in
> > a single IRQ disable.
>   I wanted to have interrupts disabled for as short as possible but if you
> think it doesn't matter, I'll take your advice. The result is attached.

Thing is, disabling interrupts is quite expensive and the extra few
instructions covered isn't much.