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

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

On Thu, 18 Jun 2020 at 21:24, Volodymyr Babchuk
<Volodymyr_Babchuk@xxxxxxxx> wrote:
>
>
> Hi Julien,
>
> Julien Grall writes:
>
> > Hi Volodymyr,
> >
> > On 12/06/2020 12:44, Volodymyr Babchuk wrote:
> >>
> >> On Fri, 2020-06-12 at 06:57 +0200, 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.
> >>
> >> Looks like arm does not support 64 bit variables. >
> >> Julien, I believe, this is armv7 limitation? Should armv8 work with 64-
> >> bit atomics?
> >
> > 64-bit atomics can work on both Armv7 and Armv8 :). It just haven't
> > been plumbed yet.
> >
> > I am happy to write a patch if you need atomic64_t or even a 64-bit
> > add_sized().
>
> Looks like I'll need this patch. So, if you still have time, it will be
> great, if you'll write it.

I offered help for either the atomic64_t or the add_sized(). Can you
confirm which one you need?

Cheers,