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

["Jürgen Groß" <jgross@suse.com>]

On 13.06.20 00:27, Volodymyr Babchuk wrote:
> On Fri, 2020-06-12 at 17:29 +0200, Dario Faggioli wrote:
>> On Fri, 2020-06-12 at 14:41 +0200, Jürgen Groß wrote:
>>> On 12.06.20 14:29, Julien Grall wrote:
>>>> On 12/06/2020 05:57, Jürgen Groß wrote:
>>>>> On 12.06.20 02:22, Volodymyr Babchuk wrote:
>>>>>> @@ -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.
>>>>
>>>> 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.
>>>
>> Yes, either percpu or atomic looks much better than locking, to me, for
>> this.
> 
> Looks like we going to have atomic64_t after all. So, I'll prefer to to
> use atomics there.

Performance would be better using percpu variables, as those would avoid
the cacheline moved between cpus a lot.


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