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

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Date:	Wed, 28 May 2014 19:51:11 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>, "H. Peter Anvin" <hpa@...or.com>,
	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>,
	Dave Hansen <dave.hansen@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 2/2] x86_64: expand kernel stack to 16K

[ Crossed emails ]

On Wed, May 28, 2014 at 6:58 PM, Dave Chinner <david@...morbit.com> wrote:
> On Thu, May 29, 2014 at 11:30:07AM +1000, Dave Chinner wrote:
>>
>> And now we have too deep a stack due to unplugging from io_schedule()...
>
> So, if we make io_schedule() push the plug list off to the kblockd
> like is done for schedule()....

We might have a few different cases.

The cases where we *do* care about latency is when we are waiting for
the IO ourselves (ie in wait_on_page() and friends), and those end up
using io_schedule() too.

So in *that* case we definitely have a latency argument for doing it
directly, and we shouldn't kick it off to kblockd. That's very much a
"get this started as soon as humanly possible".

But the "wait_iff_congested()" code that also uses io_schedule()
should push it out to kblockd, methinks.

>> This stack overflow shows us that just the memory reclaim + IO
>> layers are sufficient to cause a stack overflow,
>
> .... we solve this problem directly by being able to remove the IO
> stack usage from the direct reclaim swap path.
>
> IOWs, we don't need to turn swap off at all in direct reclaim
> because all the swap IO can be captured in a plug list and
> dispatched via kblockd. This could be done either by io_schedule()
> or a new blk_flush_plug_list() wrapper that pushes the work to
> kblockd...

That would work. That said, I personally would not mind to see that
"swap is special" go away, if possible. Because it can be behind a
filesystem too. Christ, even NFS (and people used to fight that tooth
and nail!) is back as a swap target..

                Linus
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