linux-kernel - Re: [PATCH 4/5] sched_ext: Split the global DSQ per NUMA node

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Message-ID: <20240926215644.GF26346@maniforge>
Date: Thu, 26 Sep 2024 16:56:44 -0500
From: David Vernet <void@...ifault.com>
To: Tejun Heo <tj@...nel.org>
Cc: kernel-team@...a.com, linux-kernel@...r.kernel.org, sched-ext@...a.com
Subject: Re: [PATCH 4/5] sched_ext: Split the global DSQ per NUMA node

On Tue, Sep 24, 2024 at 02:06:06PM -1000, Tejun Heo wrote:
> In the bypass mode, the global DSQ is used to schedule all tasks in simple
> FIFO order. All tasks are queued into the global DSQ and all CPUs try to
> execute tasks from it. This creates a lot of cross-node cacheline accesses
> and scheduling across the node boundaries, and can lead to live-lock
> conditions where the system takes tens of minutes to disable the BPF
> scheduler while executing in the bypass mode.
> 
> Split the global DSQ per NUMA node. Each node has its own global DSQ. When a
> task is dispatched to SCX_DSQ_GLOBAL, it's put into the global DSQ local to
> the task's CPU and all CPUs in a node only consume its node-local global
> DSQ.
> 
> This resolves a livelock condition which could be reliably triggered on an
> 2x EPYC 7642 system by running `stress-ng --race-sched 1024` together with
> `stress-ng --workload 80 --workload-threads 10` while repeatedly enabling
> and disabling a SCX scheduler.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

Acked-by: David Vernet <void@...ifault.com>

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