linux-kernel - Re: [PATCH 1/2] sched/fair: Add EAS checks before updating overutilized

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Date: Wed, 28 Feb 2024 00:45:08 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
CC: <mingo@...nel.org>, <peterz@...radead.org>, <vincent.guittot@...aro.org>,
	<dietmar.eggemann@....com>, <linux-kernel@...r.kernel.org>,
	<nysal@...ux.ibm.com>, <aboorvad@...ux.ibm.com>, <srikar@...ux.vnet.ibm.com>,
	<vschneid@...hat.com>, <pierre.gondois@....com>, <morten.rasmussen@....com>,
	<qyousef@...alina.io>
Subject: Re: [PATCH 1/2] sched/fair: Add EAS checks before updating
 overutilized

On 2024-02-23 at 20:37:06 +0530, 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.

Looks like a typical cache false sharing: CPU1 updates the rd->overutilized,
which invalid the cache line when CPU2 access adjacent rd->overload.
This changes looks good to me, just some minor questions:

> 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
> 
> While here, Fix updating overutilized as either SG_OVERUTILIZED or 0
> instead. Current code can make it 0, 1 or 2. This shouldn't alter the
> functionality.

Just wonder where 1 comes from? In current code we either write SG_OVERUTILIZED
or sg_status & SG_OVERUTILIZED.

> 
> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
>  kernel/sched/fair.c | 36 +++++++++++++++++++++++++-----------
>  1 file changed, 25 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8e30e2bb77a0..9529d9ef2c5b 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);

Is this !!status intentional? The original one is SG_OVERUTILIZED = 2,
now it is either 0 or 1.

thanks,
Chenyu