Re: [PATCH v2 1/2] sched/fair: Add EAS checks before updating overutilized
From: Shrikanth Hegde
Date: Wed Feb 28 2024 - 12:25:02 EST
On 2/28/24 9:28 PM, Pierre Gondois wrote:
Hi Pierre, Thanks for taking a look.
> It is nice to avoid calling effective_cpu_util() through the following
> when EAS is not enabled:
> I think we are avoiding calling cpu_overutilized except in update_sg_lb_stats.
I didnt want to put a EAS check in cpu_overutilized as it could be useful
function in non-EAS cases in future. calling cpu_overutilized alone doesnt
do any access to root_domain's overutilized field. So we are okay w.r.t to
cache issues.
But we will do some extra computation currently and then not use it if it
Non-EAS case in update_sg_lb_stats
Would something like this makes sense?
@@ -9925,7 +9925,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
if (nr_running > 1)
*sg_status |= SG_OVERLOAD;
- if (cpu_overutilized(i))
+ if (sched_energy_enabled() && cpu_overutilized(i))
*sg_status |= SG_OVERUTILIZED;
I didnt find how would util_fits_cpu ends up calling effective_cpu_util.
Could you please elaborate?
> cpu_overutilized()
> \-util_fits_cpu()
> \- ...
> \-effective_cpu_util()
>
> On 2/28/24 08:16, Shrikanth Hegde wrote:
>> Overutilized field of root domain is only used for EAS(energy aware
>> scheduler)
>> to decide whether to do regular load balance or EAS aware load
>> balance. It
>> is not used if EAS not possible.
>>
>> Currently enqueue_task_fair and task_tick_fair accesses, sometime updates
>> this field. In update_sd_lb_stats it is updated often.
>> Which causes cache contention due to load/store tearing and burns
>> a lot of cycles. Hence add EAS check before updating this field.
>> EAS check is optimized at compile time or it is static branch.
>> Hence it shouldn't cost much.
>>
>> With the patch, both enqueue_task_fair and newidle_balance don't show
>> up as hot routines in perf profile.
>>
>> 6.8-rc4:
>> 7.18% swapper [kernel.vmlinux] [k]
>> enqueue_task_fair
>> 6.78% s [kernel.vmlinux] [k] newidle_balance
>> +patch:
>> 0.14% swapper [kernel.vmlinux] [k]
>> enqueue_task_fair
>> 0.00% swapper [kernel.vmlinux] [k] newidle_balance
>>
>> Minor change; trace_sched_overutilized_tp expect that second argument to
>> be bool. So do a int to bool conversion for that.
>>
>> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point
>> indicator")
>> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
>> ---
>> kernel/sched/fair.c | 35 ++++++++++++++++++++++++-----------
>> 1 file changed, 24 insertions(+), 11 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 8e30e2bb77a0..3105fb08b87e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6670,15 +6670,30 @@ static inline bool cpu_overutilized(int cpu)
>> return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min,
>> rq_util_max, cpu);
>> }
>>
>> -static inline void update_overutilized_status(struct rq *rq)
>> +static inline void update_rd_overutilized_status(struct root_domain *rd,
>> + int status)
>> {
>> - if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu)) {
>> - WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED);
>> - trace_sched_overutilized_tp(rq->rd, SG_OVERUTILIZED);
>> + if (sched_energy_enabled()) {
>> + WRITE_ONCE(rd->overutilized, status);
>> + trace_sched_overutilized_tp(rd, !!status);
>> + }
>> +}
>
> NIT:
> When called from check_update_overutilized_status(),
> sched_energy_enabled() will be checked twice.
Yes.
But, I think that's okay since it is a static branch check at best.
This way it keeps the code simpler.
>
>> +
>> +static inline void check_update_overutilized_status(struct rq *rq)
>> +{
>> + /*
>> + * overutilized field is used for load balancing decisions only
>> + * if energy aware scheduler is being used
>> + */
>> + if (sched_energy_enabled()) {
>> + if (!READ_ONCE(rq->rd->overutilized) &&
>> cpu_overutilized(rq->cpu))
>> + update_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
>> }
>> }
>> #else
>> -static inline void update_overutilized_status(struct rq *rq) { }
>> +static inline void check_update_overutilized_status(struct rq *rq) { }
>> +static inline void update_rd_overutilized_status(struct root_domain *rd,
>> + bool status) { }
>> #endif
>>
>> /* Runqueue only has SCHED_IDLE tasks enqueued */
>> @@ -6779,7 +6794,7 @@ enqueue_task_fair(struct rq *rq, struct
>> task_struct *p, int flags)
>> * and the following generally works well enough in practice.
>> */
>> if (!task_new)
>> - update_overutilized_status(rq);
>> + check_update_overutilized_status(rq);
>>
>> enqueue_throttle:
>> assert_list_leaf_cfs_rq(rq);
>> @@ -10613,13 +10628,11 @@ static inline void update_sd_lb_stats(struct
>> lb_env *env, struct sd_lb_stats *sd
>> WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);
>>
>> /* Update over-utilization (tipping point, U >= 0) indicator */
>> - WRITE_ONCE(rd->overutilized, sg_status & SG_OVERUTILIZED);
>> - trace_sched_overutilized_tp(rd, sg_status & SG_OVERUTILIZED);
>> + update_rd_overutilized_status(rd, sg_status & SG_OVERUTILIZED);
>> } else if (sg_status & SG_OVERUTILIZED) {
>> struct root_domain *rd = env->dst_rq->rd;
>>
>> - WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
>> - trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
>> + update_rd_overutilized_status(rd, SG_OVERUTILIZED);
>> }
>>
>> update_idle_cpu_scan(env, sum_util);
>> @@ -12625,7 +12638,7 @@ static void task_tick_fair(struct rq *rq,
>> struct task_struct *curr, int queued)
>> task_tick_numa(rq, curr);
>>
>> update_misfit_status(curr, rq);
>> - update_overutilized_status(task_rq(curr));
>> + check_update_overutilized_status(task_rq(curr));
>>
>> task_tick_core(rq, curr);
>> }
>> --
>> 2.39.3
>>