Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed
From: Vincent Guittot
Date: Thu Feb 22 2018 - 03:38:04 EST
On 21 February 2018 at 14:13, Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
> On 02/16/2018 01:44 PM, Vincent Guittot wrote:
>> On 16 February 2018 at 13:13, Valentin Schneider
>> <valentin.schneider@xxxxxxx> wrote:
>>> On 02/14/2018 03:26 PM, Vincent Guittot wrote:
>>>> Stopped the periodic update of blocked load when all idle CPUs have fully
>>>> decayed. We introduce a new nohz.has_blocked that reflect if some idle
>>>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked
>>>> is set everytime that a Idle CPU can have blocked load and it is then clear
>>>> when no more blocked load has been detected during an update. We don't need
>>>> atomic operation but only to make cure of the right ordering when updating
>>>> nohz.idle_cpus_mask and nohz.has_blocked.
>>>>
>>>> Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>>>> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>>> ---
>>>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++---------
>>>> kernel/sched/sched.h | 1 +
>>>> 2 files changed, 102 insertions(+), 21 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 7af1fa9..5a6835e 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>>
>>>> [...]
>>>> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>>> * work being done for other cpus. Next load
>>>> * balancing owner will pick it up.
>>>> */
>>>> - if (need_resched())
>>>> - break;
>>>> + if (need_resched()) {
>>>> + has_blocked_load = true;
>>>> + goto abort;
>>>> + }
>>>>
>>>> rq = cpu_rq(balance_cpu);
>>>>
>>>
>>> I'd say it's safe to do the following here. The flag is raised in
>>> nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU
>>> that just got added to nohz.idle_cpus_mask.
>>
>> rq->has_blocked_load will be set before the barrier only if
>> nohz_tick_stopped is not already set,
>> Otherwise, we skip cpumask update and the barrier in nohz_balance_enter_idle
>>
>
> Right, forgot about that bit. I think it's still fine because:
> - nohz_balance_enter_idle() can't be called before the last running task is
> dequeued, which requires the rq lock.
> - update_blocked_averages takes the rq lock and clears rq->has_blocked_load
> with the lock held.
>
> So even though we could have some very unlikely scenario where a CPU quickly
> goes out/in of idle after nohz.idle_cpus_mask has been read, the blocked load
> itself is safe so rq->has_blocked_load will end up being set correctly.
> (Took me a while to see it that way)
>
>
> BTW, with the current set on Peter's sched/testing, update_nohz_stats()
> is called here, which doesn't do the update if !rq->has_blocked_load
> (Although that check is done without lock/barrier, so maybe we could not see
> a CPU that just went idle ?)
>
> I have one more question on that bit:
>
>
> has_blocked_load |= update_nohz_stats(rq, true);
>
> /*
> * If time for next balance is due,
> * do the balance.
> */
> if (time_after_eq(jiffies, rq->next_balance)) {
> struct rq_flags rf;
>
> rq_lock_irqsave(rq, &rf);
> update_rq_clock(rq);
> cpu_load_update_idle(rq);
> rq_unlock_irqrestore(rq, &rf);
>
> if (flags & NOHZ_BALANCE_KICK)
> rebalance_domains(rq, CPU_IDLE);
> }
>
> if (time_after(next_balance, rq->next_balance)) {
> next_balance = rq->next_balance;
> update_next_balance = 1;
> }
>
>
> Now that I think about it, shouldn't we always have a 'continue' after
> the blocked load update if (flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK ?
> AFAICT we don't want to push the next_balance forward, only the next_blocked.
But we don't push next_balance forward. It just get the shortest
next_balance and update nohz.next_balance exactly like what is done in
full idle load balance
> That would also take care of not doing the load balance.
>>>
>>> /*
>>> * This cpu doesn't have any remaining blocked load, skip it.
>>> * It's sane to do this because this flag is raised in
>>> * nohz_balance_enter_idle()
>>> */
>>> if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK &&
>>> !rq->has_blocked_load)
>>> continue;
Then, it's worth keeping the call to cpu_load_update_idle(rq); which
update the cpu_load[] array which is still used at some level
>>>
>>>> + update_blocked_averages(rq->cpu);
>>>> + has_blocked_load |= rq->has_blocked_load;
>>>> +