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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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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?

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


---
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;
--
2.31.1
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