linux-kernel - Re: [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node

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Message-ID: <3df2bf0e-0b1d-d299-3b8e-51c306cdc559@inria.fr>
Date:   Mon, 25 Mar 2019 18:45:30 +0100
From:   Brice Goglin <Brice.Goglin@...ia.fr>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        "Du, Fan" <fan.du@...el.com>, "Huang, Ying" <ying.huang@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node

Le 25/03/2019 à 17:56, Dan Williams a écrit :
>
> I'm generally against the concept that a "pmem" or "type" flag should
> indicate anything about the expected performance of the address range.
> The kernel should explicitly look to the HMAT for performance data and
> not otherwise make type-based performance assumptions.

Oh sorry, I didn't mean to have the kernel use such a flag to decide of
placement, but rather to expose more information to userspace to clarify
what all these nodes are about when userspace will decide where to
allocate things.

I understand that current NVDIMM-F are not slower than DDR and HMAT
would better describe this than a flag. But I have seen so many buggy or
dummy SLIT tables in the past that I wonder if we can expect HMAT to be
widely available (and correct).

Is there a safe fallback in case of missing or buggy HMAT? For instance,
is DDR supposed to be listed before NVDIMM (or HBM) in SRAT?

Brice