linux-kernel - Re: [PATCH 2/6] futex: Implement FUTEX2

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Message-ID: <50dc2133-5f43-3a02-38a9-234e8acb5b8c@gentwo.org>
Date: Mon, 28 Oct 2024 15:37:15 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Peter Zijlstra <peterz@...radead.org>
cc: tglx@...utronix.de, linux-kernel@...r.kernel.org, mingo@...hat.com, 
    dvhart@...radead.org, dave@...olabs.net, andrealmeid@...lia.com, 
    Andrew Morton <akpm@...ux-foundation.org>, urezki@...il.com, 
    hch@...radead.org, lstoakes@...il.com, Arnd Bergmann <arnd@...db.de>, 
    linux-api@...r.kernel.org, linux-mm@...ck.org, linux-arch@...r.kernel.org, 
    malteskarupke@....de, llong@...hat.com
Subject: Re: [PATCH 2/6] futex: Implement FUTEX2_NUMA

On Mon, 28 Oct 2024, Peter Zijlstra wrote:

> Using get_task_policy() seems very dangerous to me. It is explicitly
> possible for different tasks in a process to have different policies,
> which means (private) futexes would fail to work correctly.
>
> We need something that is process wide consistent -- like the vma
> policies. Except at current, those are to expensive to readily access.

The vma policies are bound to addresses that in turn yields address space
wide validity.

However, different threads may run on processes on different nodes and
therefore having different numa nodes close to them etc.