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

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 12/06/2020 05:57, Jürgen Groß wrote:
> On 12.06.20 02:22, Volodymyr Babchuk wrote:
> > As scheduler code now collects time spent in IRQ handlers and in
> > do_softirq(), we can present those values to userspace tools like
> > xentop, so system administrator can see how system behaves.
> >
> > We are updating counters only in sched_get_time_correction() function
> > to minimize number of taken spinlocks. As atomic_t is 32 bit wide, it
> > is not enough to store time with nanosecond precision. So we need to
> > use 64 bit variables and protect them with spinlock.
> >
> > Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
> > ---
> >   xen/common/sched/core.c     | 17 +++++++++++++++++
> >   xen/common/sysctl.c         |  1 +
> >   xen/include/public/sysctl.h |  4 +++-
> >   xen/include/xen/sched.h     |  2 ++
> >   4 files changed, 23 insertions(+), 1 deletion(-)
> >
> > diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
> > index a7294ff5c3..ee6b1d9161 100644
> > --- a/xen/common/sched/core.c
> > +++ b/xen/common/sched/core.c
> > @@ -95,6 +95,10 @@ static struct scheduler __read_mostly ops;
> >   static bool scheduler_active;
> > +static DEFINE_SPINLOCK(sched_stat_lock);
> > +s_time_t sched_stat_irq_time;
> > +s_time_t sched_stat_hyp_time;
> > +
> >   static void sched_set_affinity(
> >       struct sched_unit *unit, const cpumask_t *hard, const cpumask_t 
> > *soft);
> > @@ -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.
add_sized() is definitely not atomic. It will only prevent the compiler 
to read/write multiple time the variable.
If we expect sched_get_time_correction to be called concurrently then we 
would need to introduce atomic64_t or a spin lock.
Cheers,

--
Julien Grall