linux-kernel - Re: [PATCH] irq/affinity: Assign all CPUs a vector

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Message-ID: <6e7a93a7-b0bb-8835-6c49-3eaa3203e1d8@grimberg.me>
Date:   Wed, 29 Mar 2017 20:15:50 +0300
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Keith Busch <keith.busch@...el.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] irq/affinity: Assign all CPUs a vector


> The number of vectors to assign needs to be adjusted for each node such
> that it doesn't exceed the number of CPUs in that node. This patch
> recalculates the vector assignment per-node so that we don't try to
> assign more vectors than there are CPUs. When that previously happened,
> the cpus_per_vec was calculated to be 0, so many vectors had no CPUs
> assigned. This then goes on to fail to allocate descriptors due to
> empty masks, leading to an unoptimal spread.

Can you give a specific (numeric) example where this happens? I'm having
a little trouble following the logical change here.