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

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Message-ID: <3129d8ee-4942-cf6a-8089-15ef5a838f21@grimberg.me>
Date:   Wed, 29 Mar 2017 20:50:14 +0300
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Keith Busch <keith.busch@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] irq/affinity: Assign all CPUs a vector


> Sure, I have a 2-socket server with 16 threads each. I take one CPU
> offline in socket 2, so I've 16 threads on socket 1, 15 in socket 2. In
> total, 31 threads so requesting 31 vectors.
>
> Currently, vecs_per_node is calculated in the first iteration as 31 / 2, so 15.
>
> ncpus of socket 1 is 16. cpus_per_vec = 16 / 15, so 1 CPU per vector
> with one extra.
>
> When iterating the second socket, though, vecs_per_node is incremented
> from 15 to 16 (to account for the "extra" from before). However, the
> ncpus is only 15, so that iteration calculates:
>
>   cpus_per_vec = 15 / 16
>
> And since that's zero, the remaining 16 vectors are not assigned to any
> CPU, and the second socket has no vectors assigned to their CPUs.

Thanks for the clarification, makes sense...

Reviewed-by: Sagi Grimberg <sagi@...mberg.me>