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SubjectRe: [RFC PATCH] sched/fair: Interleave cfs bandwidth timers for improved single thread performance at low utilization
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shrikanth hegde <sshegde@linux.vnet.ibm.com> writes:

> On 2/20/23 11:08 PM, Peter Zijlstra wrote:
>> On Tue, Feb 14, 2023 at 08:54:09PM +0530, shrikanth hegde wrote:
>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index ff4dbbae3b10..7b69c329e05d 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -5939,14 +5939,25 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>>>
>>> void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>>> {
>>> - lockdep_assert_held(&cfs_b->lock);
>>> + struct hrtimer *period_timer = &cfs_b->period_timer;
>>> + s64 incr = ktime_to_ns(cfs_b->period) / 10;
>>> + ktime_t delta;
>>> + u64 orun = 1;
>>>
>>> + lockdep_assert_held(&cfs_b->lock);
>>> if (cfs_b->period_active)
>>> return;
>>>
>>> cfs_b->period_active = 1;
>>> - hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>>> - hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>>> + delta = ktime_sub(period_timer->base->get_time(),
>>> + hrtimer_get_expires(period_timer));
>>> + if (unlikely(delta >= cfs_b->period)) {
>>> + orun = ktime_divns(delta, incr);
>>> + hrtimer_add_expires_ns(period_timer, incr * orun);
>>> + }
>>> +
>>> + hrtimer_forward_now(period_timer, cfs_b->period);
>>> + hrtimer_start_expires(period_timer, HRTIMER_MODE_ABS_PINNED);
>>> }
>>
>> What kind of mad hackery is this? Why can't you do the sane thing and
>> initialize the timer at !0 in init_cfs_bandwidth(), then any of the
>> forwards will stay in period -- as they should.
>>
>> Please, go re-read Thomas's email.
>
> Thank you Peter for taking a look and review.
> we can scrap this implementation and switch to the one you suggested.
> All we need is to initialize the offset.
>
> Only reason was the way i had implemented. This implementation couldn't be
> fit into init_cfs_bandwidth as timers would align if the cgroups are
> created together and continue to align forever.
>
>>
>> *completely* untested...
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7c46485d65d7..4d6ea76096dc 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5915,6 +5915,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>>
>> INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
>> hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
>> + cfs_b->period_timer.node.expires = get_random_u32_below(cfs_b->period);
>
> This approach/implementation is better as the random function provides uniform
> distribution. Had to modify this a bit to make it work. Along with setting
> setting node.expires, we need to set _softexpires as well. Which is what
> hrtimer_set_expires does.
>
> Here are the similar numbers again.
> 8 Core system with SMT=8. Total of 64 CPU
> Workload: stress-ng --cpu=32 --cpu-ops=50000
>
> 6.2-rc6 | with patch
> 8Core 1CG power 2CG power | 1CG power 2CG power
> 27.5 80.6 40 90 | 27.3 82 32.3 104
> 27.5 81 40.2 91 | 27.5 81 38.7 96
> 27.7 80 40.1 89 | 27.6 80 29.7 115
> 27.7 80.1 40.3 94 | 27.6 80 31.5 105
>
> Will collect some more benchmarks numbers w.r.t to performance.
>
>
>> cfs_b->period_timer.function = sched_cfs_period_timer;
>> hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>> cfs_b->slack_timer.function = sched_cfs_slack_timer;
>
> This below patch worked.
> Does the below patch look okay? shall i send the [PATCH V1] with this
> change?

Yeah, this design makes way more sense.

>
> Question.
> Should we skip this adding the offset for root_task_group?

The value should never come up, so it's just a question of if it's fine
to call get_random_* in early contexts, which I don't know offhand.

>
>
> ---
> kernel/sched/fair.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff4dbbae3b10..6448533178af 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5923,6 +5923,9 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
> hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
> cfs_b->period_timer.function = sched_cfs_period_timer;
> + /* Add a random offset so that timers interleave */
> + hrtimer_set_expires(&cfs_b->period_timer, get_random_u32_below(cfs_b->period));
> +
> hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> cfs_b->slack_timer.function = sched_cfs_slack_timer;
> cfs_b->slack_started = false;

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