linux-kernel - Re: [PATCH 3/3 v6] mm/vmalloc: Cache the vmalloc memory info

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Message-ID: <87io83wiuo.fsf@rasmusvillemoes.dk>
Date:	Tue, 25 Aug 2015 16:19:59 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	George Spelvin <linux@...izon.com>, dave@...1.net,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	peterz@...radead.org, riel@...hat.com, rientjes@...gle.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 3/3 v6] mm/vmalloc: Cache the vmalloc memory info

On Tue, Aug 25 2015, Ingo Molnar <mingo@...nel.org> wrote:

> * George Spelvin <linux@...izon.com> wrote:
>
>> (I hope I'm not annoying you by bikeshedding this too much, although I
>> think this is improving.)
>
> [ I don't mind, although I wish other, more critical parts of the kernel got this
>   much attention as well ;-) ]
>

Since we're beating dead horses, let me point out one possibly
unintentional side-effect of initializing just one of vmap_info{,_cache}_gen:

$ nm -n vmlinux | grep -E 'vmap_info(_cache)?_gen'
ffffffff81e4e5e0 d vmap_info_gen
ffffffff820d5700 b vmap_info_cache_gen

[Up-thread, you wrote "I also moved the function-static cache next to the
flag and seqlock - this should further compress the cache footprint."]

One should probably ensure that they end up in the same cacheline if one
wants the fast-path to be as fast as possible - the easiest way to
ensure that is to put them in a small struct, and that might as well
contain the spinlock and the cache itself as well.

It's been fun seeing this evolve, but overall, I tend to agree with
Peter: It's a lot of complexity for little gain. If we're not going to
just kill the Vmalloc* fields (which is probably too controversial)
I'd prefer Linus' simpler version.

Rasmus
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