linux-kernel - Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

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Message-ID: <20200703215157.GI25523@casper.infradead.org>
Date:   Fri, 3 Jul 2020 22:51:57 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        peter enderborg <peter.enderborg@...y.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        GregKroah-Hartmangregkh@...uxfoundation.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

On Fri, Jul 03, 2020 at 11:20:47PM +0200, Uladzislau Rezki wrote:
> Some background:
> Actually i have been thinking about making vmalloc address space to
> be per-CPU, i.e. divide it to per-CPU address space making an allocation
> lock-less. It will eliminate a high lock contention. When i have done
> a prototype i noticed and realized that there is a silly issue with
> nr_cpu_ids on some systems.

vfree() may happen on a different CPU from the one which called vmalloc(),
so I'm not sure you're going to get as large a win as you think you will.