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

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

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.

-- 
Volodymyr Babchuk at EPAM