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

["Jürgen Groß" <jgross@suse.com>]

On 12.06.20 14:29, Julien Grall wrote:
> 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@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.
> 
> add_sized() is definitely not atomic. It will only prevent the compiler 
> to read/write multiple time the variable.

Oh, my bad, I let myself fool by it being defined in atomic.h.

> 
> 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.


Juergen