Re: [RFC PATCH v1 4/6] xentop: collect IRQ and HYP time statistics.

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.06.20 00:27, Volodymyr Babchuk wrote:
> On Fri, 2020-06-12 at 17:29 +0200, Dario Faggioli wrote:
> > On Fri, 2020-06-12 at 14:41 +0200, Jürgen Groß wrote:
> > > On 12.06.20 14:29, Julien Grall wrote:
> > > > On 12/06/2020 05:57, Jürgen Groß wrote:
> > > > > On 12.06.20 02:22, Volodymyr Babchuk wrote:
> > > > > > @@ -994,9 +998,22 @@ s_time_t sched_get_time_correction(struct
> > > > > > sched_unit *u)
> > > > > >                break;
> > > > > >        }
> > > > > > +    spin_lock_irqsave(&sched_stat_lock, flags);
> > > > > > +    sched_stat_irq_time += irq;
> > > > > > +    sched_stat_hyp_time += hyp;
> > > > > > +    spin_unlock_irqrestore(&sched_stat_lock, flags);
> > > > > Please don't use a lock. Just use add_sized() instead which will
> > > > > add
> > > > > atomically.
> > > > If we expect sched_get_time_correction to be called concurrently
> > > > then we
> > > > would need to introduce atomic64_t or a spin lock.
> > > Or we could use percpu variables and add the cpu values up when
> > > fetching the values.
> > Yes, either percpu or atomic looks much better than locking, to me, for
> > this.
> Looks like we going to have atomic64_t after all. So, I'll prefer to to
> use atomics there.
Performance would be better using percpu variables, as those would avoid
the cacheline moved between cpus a lot.


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