Re: [PATCH 3/3] mm, meminit: Allow early_pfn_to_nid to be used during runtime

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 17, 2015 at 02:39:13PM +0100, Mel Gorman wrote:

> I'm don't know and no longer have access to the necessary machine to test
> any more. You make a reasonable point and I would be surprised if it was
> noticable. On the other hand, conditional locking is evil and the patch
> reflected my thinking at the time "we don't need locks during boot". It's
> the type of thinking that should be backed with figures if it was to be
> used at all so lets go with;

Last time I tested it, an uncontended spinlock (cache hot) ran around 20
cycles, the unlock is a regular store (x86) and in single digit cycles.
I doubt modern hardware makes it go slower.

> ---8<---
> mm, meminit: Allow early_pfn_to_nid to be used during runtime v2
> 
> early_pfn_to_nid historically was inherently not SMP safe but only
> used during boot which is inherently single threaded or during hotplug
> which is protected by a giant mutex. With deferred memory initialisation
> there was a thread-safe version introduced and the early_pfn_to_nid
> would trigger a BUG_ON if used unsafely. Memory hotplug hit that check.
> This patch makes early_pfn_to_nid introduces a lock to make it safe to
> use during hotplug.
> 
> Reported-and-tested-by: Alex Ng <alexng@microsoft.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

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