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

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Julien,

Julien Grall writes:

> On Thu, 18 Jun 2020 at 21:24, Volodymyr Babchuk
> <Volodymyr_Babchuk@epam.com> 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@epam.com>
>> >>>> ---
>> >>>>    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?

Yes, sorry. I had atomic64_t in mind.

-- 
Volodymyr Babchuk at EPAM