linux-kernel - Re: [PATCH v2] mm: per-thread vma caching

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Date:	Tue, 25 Feb 2014 10:45:51 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: per-thread vma caching

On Tue, Feb 25, 2014 at 10:37 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>  - clear all the cache entries (of the new 'struct task_struct'! - so
> not in dup_mmap, but make sure it's zeroed when allocating!)(
>
>  - set vmcache_seqnum to 0 in dup_mmap (since any sequence number is
> fine when it got invalidated, and 0 is best for "avoid overflow").

Btw, as far as I can tell, that also makes the per-thread vmacache
automatically do the right thing for the non-MMU case, so that you
could just remove the difference between CONFIG_MMU and NOMMU.

Basically, dup_mmap() should no longer have anything to do with the
vmacache, since it is now per-thread, not per-mm.

So :

 - allocating a new "struct mm_struct" should clear the
vmacache_seqnum for that new mm, to try to minimize unnecessary future
overflow.

 - thread allocation should just zero the cache entries, and set
"tsk->vmacache_seqnum = mm->vmacache_seqnum" (after dup_mm()) to avoid
future unnecessary flushes.

and as far as I can tell, the logic would be exactly the same on NOMMU
(the dup_mm just doesn't happen, since all forks are basically sharing
mm).

And maybe you'd want to make VMACACHE_SIZE be 1 on NOMMU (and make
sure to change the "& 3" to "& (VMACACHE_SIZE-1)". Just to keep the
size down on small systems that really don't need it.

          Linus
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