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From: Peter Zijlstra <peterz@infradead.org>
To: Lauro Ramos Venancio <lvenanci@redhat.com>
Cc: linux-kernel@vger.kernel.org, lwang@redhat.com, riel@redhat.com,
	Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology
Date: Thu, 13 Apr 2017 17:48:12 +0200
Message-ID: <20170413154812.vrtkdyzgkrywj2no@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1492091769-19879-3-git-send-email-lvenanci@redhat.com>

On Thu, Apr 13, 2017 at 10:56:08AM -0300, Lauro Ramos Venancio wrote:
> Currently, on a 4 nodes NUMA machine with ring topology, two sched
> groups are generated for the last NUMA sched domain. One group has the
> CPUs from NUMA nodes 3, 0 and 1; the other group has the CPUs from nodes
> 1, 2 and 3. As CPUs from nodes 1 and 3 belongs to both groups, the
> scheduler is unable to directly move tasks between these nodes. In the
> worst scenario, when a set of tasks are bound to nodes 1 and 3, the
> performance is severely impacted because just one node is used while the
> other node remains idle.

I feel a picture would be ever so much clearer.

> This patch constructs the sched groups from each CPU perspective. So, on
> a 4 nodes machine with ring topology, while nodes 0 and 2 keep the same
> groups as before [(3, 0, 1)(1, 2, 3)], nodes 1 and 3 have new groups
> [(0, 1, 2)(2, 3, 0)]. This allows moving tasks between any node 2-hops
> apart.

So I still have no idea what specifically goes wrong and how this fixes
it. Changelog is impenetrable.

"From each CPU's persepective" doesn't really help, there already is a
for_each_cpu() in.

Also, since I'm not sure what happend to the 4 node system, I cannot
begin to imagine what would happen on the 8 node one.

  parent reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-13 13:56 [RFC 0/3] " Lauro Ramos Venancio
2017-04-13 13:56 ` [RFC 1/3] sched/topology: Refactor function build_overlap_sched_groups() Lauro Ramos Venancio
2017-04-13 14:50   ` Rik van Riel
2017-05-15  9:02   ` [tip:sched/core] " tip-bot for Lauro Ramos Venancio
2017-04-13 13:56 ` [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology Lauro Ramos Venancio
2017-04-13 15:16   ` Rik van Riel
2017-04-13 15:48   ` Peter Zijlstra [this message]
2017-04-13 20:21     ` Lauro Venancio
2017-04-13 21:06       ` Lauro Venancio
2017-04-13 23:38         ` Rik van Riel
2017-04-14 10:48           ` Peter Zijlstra
2017-04-14 11:38   ` Peter Zijlstra
2017-04-14 12:20     ` Peter Zijlstra
2017-05-15  9:03       ` [tip:sched/core] sched/fair, cpumask: Export for_each_cpu_wrap() tip-bot for Peter Zijlstra
2017-05-17 10:53         ` hackbench vs select_idle_sibling; was: " Peter Zijlstra
2017-05-17 12:46           ` Matt Fleming
2017-05-17 14:49           ` Chris Mason
2017-05-19 15:00           ` Matt Fleming
2017-06-05 13:00             ` Matt Fleming
2017-06-06  9:21               ` Peter Zijlstra
2017-06-09 17:52                 ` Chris Mason
2017-06-08  9:22           ` [tip:sched/core] sched/core: Implement new approach to scale select_idle_cpu() tip-bot for Peter Zijlstra
2017-04-14 16:58     ` [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology Peter Zijlstra
2017-04-17 14:40       ` Lauro Venancio
2017-04-13 13:56 ` [RFC 3/3] sched/topology: Different sched groups must not have the same balance cpu Lauro Ramos Venancio
2017-04-13 15:27   ` Rik van Riel
2017-04-14 16:49   ` Peter Zijlstra
2017-04-17 15:34     ` Lauro Venancio
2017-04-18 12:32       ` Peter Zijlstra

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