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

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Date:	Tue, 03 Jun 2014 15:28:22 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.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

Possibly stupid question: Is it true that any given task can only be
using one wait_queue_t at a time? If so, would it be an idea to put a
wait_queue_t into struct task_struct [maybe union'ed with a struct
wait_bit_queue] and avoid allocating this 40 byte structure repeatedly
on the stack.

E.g., in one of Minchan's stack traces, there are two calls of
mempool_alloc (which itself declares a wait_queue_t) and one
try_to_free_pages (which is the only caller of throttle_direct_reclaim,
which in turn uses wait_event_interruptible_timeout and
wait_event_killable).

Rasmus
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