linux-kernel - Re: [PATCH RFC v1] mm: is_mem_section

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Message-ID: <20200122190903.GD29276@dhcp22.suse.cz>
Date:   Wed, 22 Jan 2020 20:09:03 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Anshuman Khandual <anshuman.khandual@....com>,
        lantianyu1986@...il.com,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

On Wed 22-01-20 19:46:15, David Hildenbrand wrote:
> On 22.01.20 19:38, Michal Hocko wrote:
[...]
> > How exactly is check + offline more optimal then offline which makes
> > check as its first step? I will get to your later points after this is
> > clarified.
> 
> Scanning (almost) lockless is more efficient than bouncing back and
> forth with the device_hotplug_lock, mem_hotplug_lock, cpu_hotplug_lock
> and zone locks - as far as I understand.

All but the zone lock shouldn't be really contended and as such
shouldn't cause any troubles. zone->lock really depends on the page
allocator usage of course. But as soon as we have a contention then it
is just more likely that the result is less reliable.

I would be also really curious about how much actual time could be saved
by this - some real numbers - because hotplug operations shouldn't
happen so often that this would stand out. At least that is my
understanding.

> And as far as I understood, that was the whole reason of the original
> commit.

Well, I have my doubts but it might be just me and I might be wrong. My
experience from a large part of the memory hotplug functionality is that
it was driven by a good intention but without a due diligence to think
behind the most obvious usecase. Having a removable flag on the memblock
sounds like a neat idea of course. But an inherently racy flag is just
borderline useful.

Anyway, I will stop at this moment and wait for real usecases.

Thanks!
-- 
Michal Hocko
SUSE Labs