From: Uladzislau Rezki <urezki@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
peter enderborg <peter.enderborg@sony.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
GregKroah-Hartmangregkh@linuxfoundation.org,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)
Date: Fri, 3 Jul 2020 23:20:47 +0200 [thread overview]
Message-ID: <20200703212047.GA6856@pc636> (raw)
In-Reply-To: <CAHk-=wiagrzJs9OBe_6bHK+Cc6RdoCOV85CiJAd3MhYnc8idfw@mail.gmail.com>
> On Fri, Jul 3, 2020 at 12:28 PM Uladzislau Rezki <urezki@gmail.com> wrote:
> >
> > I have MSI TRX40 with latest BIOS.
>
> I think it's just that the BIOS is set for the max possible, in case
> you'd have a 3990X.
>
3990x is the top one in this series, so indeed it can be a case and
explanation why nr_cpu_ids is set to 128.
>
> I compile my kernel with CONFIG_NR_CPUS's set to 64. That works around
> the issue.
>
> Lots of distros seem to set CONFIG_MAXSMP to true, which I guess is
> the most generic thing to do, but the problem with that is not just
> the silly problem with the BIOS, but it also means that the kernel
> does dynamic allocation for cpumasks even if you _don't_ have that
> problem, because at compile-time you don't know how big the cpumask
> will be.
>
> With CONFIG_NR_CPUS's set to 64, the kernel will just use a "unsigned
> long" on the stack (and in various data structures) and be done with
> it, and not do unnecessary dynamic allocations.
>
Thanks for proposed workaround! I will update the CONFIG_NR_CPUS with
proper value in my .config
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.
Therefore i reported about it.
Thanks, Linus!
--
Vlad Rezki
next prev parent reply other threads:[~2020-07-03 21:21 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-03 15:57 nr_cpu_ids vs AMD 3970x(32 physical CPUs) Uladzislau Rezki
2020-07-03 16:56 ` Peter Zijlstra
2020-07-03 17:09 ` Uladzislau Rezki
2020-07-03 17:38 ` Peter Zijlstra
2020-07-03 19:26 ` Uladzislau Rezki
2020-07-03 17:07 ` Gabriel C
2020-07-03 17:24 ` Uladzislau Rezki
2020-07-03 17:45 ` Peter Zijlstra
2020-07-03 18:36 ` Gabriel C
2020-07-03 19:05 ` peter enderborg
2020-07-03 19:28 ` Uladzislau Rezki
2020-07-03 20:08 ` Linus Torvalds
2020-07-03 21:20 ` Uladzislau Rezki [this message]
2020-07-03 21:51 ` Matthew Wilcox
2020-07-03 22:22 ` Uladzislau Rezki
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