linux-kernel - Re: [PATCH] percpu/module resevation: change resevation size iff X86

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Date:   Thu, 4 Apr 2019 20:53:28 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Barret Rhoden <brho@...gle.com>
cc:     Dennis Zhou <dennis@...nel.org>, Eial Czerwacki <eial@...lemp.com>,
        tj@...nel.org, linux-kernel@...r.kernel.org,
        Shai Fultheim <shai@...lemp.com>,
        Oren Twaig <oren@...lemp.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH] percpu/module resevation: change resevation size iff
 X86_VSMP is set

On Wed, 13 Mar 2019, Barret Rhoden wrote:

> > It is very expensive. VMSP exchanges 4K segments via RDMA between servers
> > to build a large address space and run a kernel in the large address
> > space. Using smaller segments can cause a lot of
> > "cacheline" bouncing (meaning transfers of 4K segments back and forth
> > between servers).
> >
>
> Given that these are large machines, would it be OK to statically reserve 64K
> on them for modules' percpu data?

Likely.

> The bug that led me to here was from someone running on a non-VSMP machine but
> had that config set.  Perhaps we make it more clear in the Kconfig option to
> not set it on other machines.  That might make it less likely anyone on a
> non-VSMP machine pays the 64K overhead.

Right.

> Are there any other alternatives?  Not using static SRCU in any code that
> could be built as a module seems a little harsh.

Sorry this ended up in my spam folder somehow. Just fished it out.