linux-kernel - Re: [PATCH v1] mm: initialize pages on demand during boot

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Message-Id: <20180112153734.1780ccc00ebced508fad397a@linux-foundation.org>
Date:   Fri, 12 Jan 2018 15:37:34 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        mgorman@...hsingularity.net, mgorman@...e.de, mhocko@...e.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm: initialize pages on demand during boot

On Fri, 12 Jan 2018 13:34:05 -0500 Pavel Tatashin <pasha.tatashin@...cle.com> wrote:

> Deferred page initialization allows the boot cpu to initialize a small
> subset of the system's pages early in boot, with other cpus doing the rest
> later on.
> 
> It is, however, problematic to know how many pages the kernel needs during
> boot.  Different modules and kernel parameters may change the requirement,
> so the boot cpu either initializes too many pages or runs out of memory.
> 
> To fix that, initialize early pages on demand.  This ensures the kernel
> does the minimum amount of work to initialize pages during boot and leaves
> the rest to be divided in the multithreaded initialization path
> (deferred_init_memmap).
> 
> The on-demand code is permanently disabled using static branching once
> deferred pages are initialized.  After the static branch is changed to
> false, the overhead is up-to two branch-always instructions if the zone
> watermark check fails or if rmqueue fails.

Presumably this fixes some real-world problem which someone has observed?

Please describe that problem for us in lavish detail.