linux-kernel - Re: [RFC 2/2] x86_64: expand kernel stack to 16K

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Message-ID: <5388C9F9.3000105@zytor.com>
Date:	Fri, 30 May 2014 11:12:09 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Dave Chinner <david@...morbit.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Jones <davej@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	PJ Waskiewicz <pjwaskiewicz@...il.com>
Subject: Re: [RFC 2/2] x86_64: expand kernel stack to 16K

On 05/30/2014 10:24 AM, Dave Hansen wrote:
> On 05/30/2014 09:06 AM, Linus Torvalds wrote:
>> On Fri, May 30, 2014 at 8:52 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>>> That said, it's still likely a non-production option due to the page
>>>> table games we'd have to play at fork/clone time.
>>>
>>> Still, seems much more tractable.
>>
>> We might be able to make it more attractive by having a small
>> front-end cache of the 16kB allocations with the second page unmapped.
>> That would at least capture the common "lots of short-lived processes"
>> case without having to do kernel page table work.
> 
> If we want to use 4k mappings, we'd need to move the stack over to using
> vmalloc() (or at least be out of the linear mapping) to avoid breaking
> up the linear map's page tables too much.  Doing that, we'd actually not
> _have_ to worry about fragmentation, and we could actually utilize the
> per-cpu-pageset code since we'd could be back to using order-0 pages.
> So it's at least not all a loss.  Although, I do remember playing with
> 4k stacks back in the 32-bit days and not getting much of a win with it.
> 
> We'd definitely that cache, if for no other reason than the vmalloc/vmap
> code as-is isn't super-scalable.
> 

I don't think we want to use 4K mappings for production...

	-hpa

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