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Date:   Fri, 11 Aug 2023 09:11:21 +0800
From:   Chen Yu <yu.c.chen@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     kernel test robot <oliver.sang@intel.com>,
        <oe-lkp@lists.linux.dev>, <lkp@intel.com>,
        <linux-kernel@vger.kernel.org>, <x86@kernel.org>,
        "Ingo Molnar" <mingo@kernel.org>
Subject: Re: [tip:sched/eevdf] [sched/fair]  e0c2ff903c:
 phoronix-test-suite.blogbench.Write.final_score -34.8% regression
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On 2023-08-10 at 21:24:37 +0800, kernel test robot wrote:
> 
> 
> Hello,
> 
> kernel test robot noticed a -34.8% regression of phoronix-test-suite.blogbench.Write.final_score on:
> 
> 
> commit: e0c2ff903c320d3fd3c2c604dc401b3b7c0a1d13 ("sched/fair: Remove sched_feat(START_DEBIT)")
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git sched/eevdf
> 
> testcase: phoronix-test-suite
> test machine: 96 threads 2 sockets Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz (Cascade Lake) with 512G memory
> parameters:
> 
> 	test: blogbench-1.1.0
> 	option_a: Write
> 	cpufreq_governor: performance
> 

It seems that commit e0c2ff903c32 removed the sched_feat(START_DEBIT) for initial
task, but also increases the vruntime for non-initial task:
Before the e0c2ff903c32, the vruntime for a enqueued task is:
cfs_rq->min_vruntime
After the e0c2ff903c32, the vruntime for a enqueued task is:
avg_vruntime(cfs_rq) = \Sum v_i * w_i / W
                     = \Sum v_i / nr_tasks
which is usually higher than cfs_rq->min_vruntime, and we give less sleep bonus to
the wakee, which could bring more or less impact to different workloads.
But since later we switched to lag based placement, this new vruntime will minus
lag, which could mitigate this problem. 


thanks,
Chenyu