linux-kernel - Re: virtio ring cleanups, which save stack on older gcc

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Date:	Thu, 29 May 2014 16:41:17 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.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>, "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>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: virtio ring cleanups, which save stack on older gcc

Hello Rusty,

On Thu, May 29, 2014 at 04:56:41PM +0930, Rusty Russell wrote:
> They don't make much difference: the easier fix is use gcc 4.8
> which drops stack required across virtio block's virtio_queue_rq
> down to that kmalloc in virtio_ring from 528 to 392 bytes.
> 
> Still, these (*lightly tested*) patches reduce to 432 bytes,
> even for gcc 4.6.4.  Posted here FYI.

I am testing with below which was hack for Dave's idea so don't have
a machine to test your patches until tomorrow.
So, I will queue your patches into testing machine tomorrow morning.

Thanks!

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f5c6635b806c..95f169e85dbe 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4241,10 +4241,13 @@ EXPORT_SYMBOL_GPL(yield_to);
 void __sched io_schedule(void)
 {
 	struct rq *rq = raw_rq();
+	struct blk_plug *plug = current->plug;
 
 	delayacct_blkio_start();
 	atomic_inc(&rq->nr_iowait);
-	blk_flush_plug(current);
+	if (plug)
+		blk_flush_plug_list(plug, true);
+
 	current->in_iowait = 1;
 	schedule();
 	current->in_iowait = 0;

> 
> Cheers,
> Rusty.
> 
> --
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-- 
Kind regards,
Minchan Kim
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