linux-kernel - Re: [RFC PATCH v2] sched/fair: search a task from the tail of the queue

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Date:   Wed, 4 Oct 2017 11:46:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Mike Galbraith <efault@....de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Paul Turner <pjt@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Kirill Tkhai <tkhai@...dex.ru>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [RFC PATCH v2] sched/fair: search a task from the tail of the
 queue

On Wed, Sep 13, 2017 at 12:24:30PM +0200, Uladzislau Rezki (Sony) wrote:
> From: Uladzislau Rezki <urezki@...il.com>
> 
> As a first step this patch makes cfs_tasks list as MRU one.
> It means, that when a next task is picked to run on physical
> CPU it is moved to the front of the list.
> 
> Therefore, the cfs_tasks list is more or less sorted (except
> woken tasks) starting from recently given CPU time tasks toward
> tasks with max wait time in a run-queue, i.e. MRU list.
> 
> Second, as part of the load balance operation, this approach
> starts detach_tasks()/detach_one_task() from the tail of the
> queue instead of the head, giving some advantages:
> 
> - tends to pick a task with highest wait time;
> - tasks located in the tail are less likely cache-hot,
>   therefore the can_migrate_task() decision is higher.
> 
> hackbench illustrates slightly better performance. For example
> doing 1000 samples and 40 groups on i5-3320M CPU, it shows below
> figures:
> 
> default: 0.657 avg
> patched: 0.646 avg
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>

Thanks!