Re: [Xen-devel] [RFC 1/9] schedule: Introduce per-pcpu time accounting

[Andrii Anisov <andrii.anisov@gmail.com>]

Hello Volodymyr,

On 11.09.19 21:01, Volodymyr Babchuk wrote:
>> Introduce per-pcpu time accounting what includes the following states:
>>
>> TACC_HYP - the pcpu executes hypervisor code like softirq processing
>>             (including scheduling), tasklets and context switches
>> TACC_GUEST - the pcpu executes guests code
>> TACC_IDLE - the low-power state of the pcpu
> Is it really low-power?

It is rather matter of scheduling design. It differs from OS to OS, even from arch to arch. See [1].
Me personally tend to treat only low-power state as a true idle.
As a bad (IMO) example I can give the current XEN mainline. Pretty heavy tasks could be performed by the idle vcpu, and they are accounted as idle. This may mislead, for example, cpufreq governor.

>> TACC_IRQ - the pcpu performs interrupts processing, without separation to
>>             guest or hypervisor interrupts
> I think, word "distinguishing" would be better than "separation"

Why so?

> 
>> TACC_GSYNC - the pcpu executes hypervisor code to process synchronous trap
>>               from the guest. E.g. hypercall processing or io emulation.
>>
>> Currently, the only reenterant state is TACC_IRQ. It is assumed, no changes
>> to state other than TACC_IRQ could happen until we return from nested
>> interrupts. IRQ time is accounted in a distinct way comparing to other states.
>> It is acumulated between other states transition moments, and is substracted
>> from the old state on states transion calculation.
>>
>> Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
>> ---
>>   xen/common/schedule.c   | 81 +++++++++++++++++++++++++++++++++++++++++++++++++
>>   xen/include/xen/sched.h | 27 +++++++++++++++++
>>   2 files changed, 108 insertions(+)
>>
>> diff --git a/xen/common/schedule.c b/xen/common/schedule.c
>> index 7b71581..6dd6603 100644
>> --- a/xen/common/schedule.c
>> +++ b/xen/common/schedule.c
>> @@ -1539,6 +1539,87 @@ static void schedule(void)
>>       context_switch(prev, next);
>>   }
>>   
>> +DEFINE_PER_CPU(struct tacc, tacc);
>> +
>> +static void tacc_state_change(enum TACC_STATES new_state)
>> +{
>> +    s_time_t now, delta;
>> +    struct tacc* tacc = &this_cpu(tacc);
>> +    unsigned long flags;
>> +
>> +    local_irq_save(flags);
>> +
>> +    now = NOW();
>> +    delta = now - tacc->state_entry_time;
>> +
>> +    /* We do not expect states reenterability (at least through this function)*/
>> +    ASSERT(new_state != tacc->state);
>> +
>> +    tacc->state_time[tacc->state] += delta - tacc->irq_time;
>> +    tacc->state_time[TACC_IRQ] += tacc->irq_time;
>> +    tacc->irq_time = 0;
>> +    tacc->state = new_state;
>> +    tacc->state_entry_time = now;
>> +
>> +    local_irq_restore(flags);
>> +}
>> +
>> +void tacc_hyp(int place)
> I believe, you want some enum for this "place" parameter type
>> +{
>> +//    printk("\ttacc_hyp %u, place %d\n", smp_processor_id(), place);
> Please, don't push commented-out code. BTW, I think, it is possible to
> add some TACC_DEBUG facilities to enable/disable this traces during
> compile-time.
> 
> Also, looks like you don't use "place" parameter at all.

Since that is the RFC, I've comforted myself with leaving my debug code in place. I hope it should not be confusing.

> 
> Lastly, I believe that this function (and other similar functions below)
> can be defined as "static inline" in a header file

Not this time. They are mostly called from asm (at least now).

> 
>> +    tacc_state_change(TACC_HYP);
>> +}
>> +
>> +void tacc_guest(int place)
>> +{
>> +//    printk("\ttacc_guest %u, place %d\n", smp_processor_id(), place);
>> +    tacc_state_change(TACC_GUEST);
>> +}
>> +
>> +void tacc_idle(int place)
>> +{
>> +//    printk("\tidle cpu %u, place %d\n", smp_processor_id(), place);
>> +    tacc_state_change(TACC_IDLE);
>> +}
>> +
>> +void tacc_gsync(int place)
>> +{
>> +//    printk("\ttacc_gsync %u, place %d\n", smp_processor_id(), place);
>> +    tacc_state_change(TACC_GSYNC);
>> +}
>> +
>> +void tacc_irq_enter(int place)
>> +{
>> +    struct tacc* tacc = &this_cpu(tacc);
>> +
>> +//    printk("\ttacc_irq_enter %u, place %d, cnt %d\n", smp_processor_id(), place, this_cpu(tacc).irq_cnt);
>> +    ASSERT(!local_irq_is_enabled());
>> +    ASSERT(tacc->irq_cnt >= 0);
> You can make irq_cnt unsigned and drop this assert.

No. Otherwise one might miss proper call sequence when utilize this for the different arch, and have no notice from the debug assertion.

> 
>> +
>> +    if ( tacc->irq_cnt == 0 )
>> +    {
>> +        tacc->irq_enter_time = NOW();
>> +    }
> Coding style:
> 
> ---
> Braces should be omitted for blocks with a single statement. e.g.,
> 
> if ( condition )
>      single_statement();
> ---
> 

OK.

>> +
>> +    tacc->irq_cnt++;
>> +}
>> +
>> +void tacc_irq_exit(int place)
>> +{
>> +    struct tacc* tacc = &this_cpu(tacc);
>> +
>> +//    printk("\ttacc_irq_exit %u, place %d, cnt %d\n", smp_processor_id(), place, tacc->irq_cnt);
>> +    ASSERT(!local_irq_is_enabled());
>> +    ASSERT(tacc->irq_cnt > 0);
>> +    if ( tacc->irq_cnt == 1 )
>> +    {
>> +        tacc->irq_time = NOW() - tacc->irq_enter_time;
>> +        tacc->irq_enter_time = 0;
>> +    }
>> +
>> +    tacc->irq_cnt--;
> What if, you IRQ will arrive right after this? I believe, you will lose
> some of the accumulated time.

See ASSERT(!local_irq_is_enabled()) above.

> 
>> +}
>> +
>>   void context_saved(struct vcpu *prev)
>>   {
>>       /* Clear running flag /after/ writing context to memory. */
>> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
>> index e3601c1..04a8724 100644
>> --- a/xen/include/xen/sched.h
>> +++ b/xen/include/xen/sched.h
>> @@ -1002,6 +1002,33 @@ extern void dump_runq(unsigned char key);
>>   
>>   void arch_do_physinfo(struct xen_sysctl_physinfo *pi);
>>   
>> +enum TACC_STATES {
> If I remember correct, enum names should in lower case

Ugh...

> 
>> +    TACC_HYP = 0,
>> +    TACC_GUEST = 1,
>> +    TACC_IDLE = 2,
>> +    TACC_IRQ = 3,
>> +    TACC_GSYNC = 4,
>> +    TACC_STATES_MAX
>> +};
>> +
>> +struct tacc
>> +{
>> +    s_time_t state_time[TACC_STATES_MAX];
>> +    s_time_t state_entry_time;
>> +    int state;
> enum, maybe?

Maybe.

> 
>> +
>> +    s_time_t guest_time;
>> +
>> +    s_time_t irq_enter_time;
>> +    s_time_t irq_time;
>> +    int irq_cnt;
>> +};
>> +
>> +DECLARE_PER_CPU(struct tacc, tacc);
>> +
>> +void tacc_hyp(int place);
>> +void tacc_idle(int place);
> What about functions from sched.c? Should they be declared there?

Maybe.

> 
>> +
>>   #endif /* __SCHED_H__ */
>>   
>>   /*
> 
> 

[1] https://elixir.bootlin.com/linux/latest/source/kernel/sched/cputime.c#L429

-- 
Sincerely,
Andrii Anisov.

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