linux-kernel - Re: [RFC PATCH 0/5] Memory access profiler(IBS) driven NUMA balancing

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Date:   Wed, 8 Feb 2023 19:03:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bharata B Rao <bharata@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, mgorman@...e.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
        tglx@...utronix.de, yue.li@...verge.com,
        Ravikumar.Bangoria@....com, ying.huang@...el.com
Subject: Re: [RFC PATCH 0/5] Memory access profiler(IBS) driven NUMA balancing

On Wed, Feb 08, 2023 at 01:05:28PM +0530, Bharata B Rao wrote:

> - Perf uses IBS and we are using the same IBS for access profiling here.
>   There needs to be a proper way to make the use mutually exclusive.

No, IFF this lives it needs to use in-kernel perf.

> - Is tying this up with NUMA balancing a reasonable approach or
>   should we look at a completely new approach?

Is it giving sufficient win to be worth it, afaict it doesn't come even
close to justifying it.

> - Hardware provided access information could be very useful for driving
>   hot page promotion in tiered memory systems. Need to check if this
>   requires different tuning/heuristics apart from what NUMA balancing
>   already does.

I think Huang Ying looked at that from the Intel POV and I think the
conclusion was that it doesn't really work out. What you need is
frequency information, but the PMU doesn't really give you that. You
need to process a *ton* of PMU data in-kernel.