linux-kernel - Re: [PATCH v2 for-4.12-fixes 1/2] sched/fair: Use task_groups instead of leaf_cfs_rq_list to walk all cfs

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Date:   Wed, 24 May 2017 16:40:34 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com,
        mohini.narkhede@...el.com
Subject: Re: [PATCH v2 for-4.12-fixes 1/2] sched/fair: Use task_groups instead
 of leaf_cfs_rq_list to walk all cfs_rqs



On 05/09/2017 09:17 AM, Tejun Heo wrote:
> Currently, rq->leaf_cfs_rq_list is a traversal ordered list of all
> live cfs_rqs which have ever been active on the CPU; unfortunately,
> this makes update_blocked_averages() O(total number of CPU cgroups)
> which isn't scalable at all.
> 
> The next patch will make rq->leaf_cfs_rq_list only contain the cfs_rqs
> which are currently active.  In preparation, this patch converts users
> which need to traverse all cfs_rqs to use task_groups list instead.
> 
> task_groups list is protected by its own lock and allows RCU protected
> traversal and the order of operations guarantees that all online
> cfs_rqs will be visited, but holding rq->lock won't protect against
> iterating an already unregistered cfs_rq.  However, the operations of
> the two users that get converted - update_runtime_enabled() and
> unthrottle_offline_cfs_rqs() - should be safe to perform on already
> dead cfs_rqs, so adding rcu read protection around them should be
> enough.
> 
> Note that print_cfs_stats() is not converted.  The next patch will
> change its behavior to print out only active cfs_rqs, which is
> intended as there's not much point in printing out idle cfs_rqs.
> 
> v2: Dropped strong synchronization around removal and left
>      print_cfs_stats() unchanged as suggested by Peterz.
> 
> 

Tejun,

We did some preliminary testing of this patchset for a well
known database benchmark on a 4 socket Skylake server system.
It provides a 3.7% throughput boost which is significant for
this benchmark.

Thanks.

Tim