linux-kernel - Re: [PATCH 0/2] avoid spurious blocked load update

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Date:   Tue, 16 Nov 2021 15:48:40 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Cc:     joel@...lfernandes.org
Subject: Re: [PATCH 0/2] avoid spurious blocked load update

On Fri, 2021-11-12 at 10:58 +0100, Vincent Guittot wrote:
> This patchset is a follow up of : 
> https://lore.kernel.org/lkml/20211019123537.17146-1-vincent.guittot@linaro.org/
> 
> It ensures that newly idle load balance will not kick the update of
> blocked load if it skips the load balance because avg_idle is too
> short.
> It also makes sure that rq->next_balance doesn't go in the past when
> updated.
> 
> Tim Chen (1):
>   sched: sched: Fix rq->next_balance time updated to earlier than
>     current time
> 
> Vincent Guittot (1):
>   sched/fair: skip newidle update stats
> 
>  kernel/sched/fair.c | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 

Vincent,

Got some data back from the benchmark team.
To my surprise, the skip_new_idle_update_stats patch
actually makes things a little worse.

                                        Relative Performance 
                                        (higher better)
5.15 rc4 vanilla (cgroup disabled)      100%
5.15 rc4 vanilla (cgroup enabled)       96%
patch v2                                96%
patch v3                                96%
patch v3
	+skip_new_idle_update_stats	93.7%
patch v3
	+skip_new_idle_update_stats
	+Fix rq->next_balance_time	93.7%	

The cpu utilization actually is the similar compared with
having just the v3 patch. In both cases they are
81% user
12% kernel
2%  idle
5%  waiting for IO


Profile on key functions
in load balancing shows a little more cpu utilization,
which is unexpected as we are cutting short
the newidle_balance.

patch v3
     0.56%          [k] __update_load_avg_cfs_rq  
     0.51%          [k] update_load_avg    
     0.39%          [k] update_blocked_averages
     0.36%          [k] __update_load_avg_se  
     0.05%	    [k] newidle_balance

patch v3 + skip_new_idle_update_stats
     0.58%          [k] __update_load_avg_cfs_rq  
     0.53%          [k] update_load_avg    
     0.40%          [k] update_blocked_averages
     0.37%          [k] __update_load_avg_se  
     0.06%	    [k] newidle_balance

Context switch frequency is lower by 4% with the skip_new_idle_update_stats
patch.  

Thanks.

Tim