linux-kernel - Re: [PATCH v6] workqueue: Fix edge cases for calc of pool's cpumask

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Message-ID: <20170727204632.GL742618@devbig577.frc2.facebook.com>
Date:   Thu, 27 Jul 2017 16:46:32 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, nfont@...ux.vnet.ibm.com
Subject: Re: [PATCH v6] workqueue: Fix edge cases for calc of pool's cpumask

Hello, Michael.

On Thu, Jul 27, 2017 at 03:15:47PM -0500, Michael Bringmann wrote:
> There is an underlying assumption in many layers / modules of the Linux
> system that CPU <-> node mapping is static.  This is despite the presence
> of features like NUMA and 'hotplug' that support the dynamic addition/
> removal of fundamental system resources like CPUs and memory.  PowerPC
> systems, however, do provide extensive features for the dynamic change
> of resources available to a system.

The text can go as-is but keeping cpu <-> node mapping static is a
trade-off rather than upper layers missing out something.  Making cpu
<-> node mapping dynamic means adding complexities and overhead to a
lot hotter paths including memory allocation.  It's just a better
trade off to keep the mapping static from arch side even if that means
we have to use more complex mapping in arch and/or allocate more
possible cpus than strictly necessary.

> Currently, there is little or no synchronization protection around the
> updating of the CPU <-> node mapping, and the export/update of this
> information for other layers / modules.  In systems which can change 
> this mapping during 'hotplug', like PowerPC, the information is changing 
> underneath all layers that might reference it.
> 
> This patch attempts to ensure that a valid, usable cpumask attribute is
> used by the workqueue infrastructure when setting up new resource pools.
> It prevents a crash that has been observed when an 'empty' cpumask is
> passed along to the worker/task scheduling code.  It is intended as an
> intermediate fix until a more fundamental review and correction of the
> issue can be done.

Thanks a lot for your patience!  Much appreciated.

-- 
tejun