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

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Date:   Fri, 1 Mar 2019 21:54:21 +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 Fri, 1 Mar 2019, Barret Rhoden wrote:

> I'm not familiar with VSMP - how bad is it to use L1 cache alignment instead
> of 4K page alignment?  Maybe some structures can use the smaller alignment?
> Or maybe have VSMP require SRCU-using modules to be built-in?

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).