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

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Message-ID: <87fvjs7sge.fsf@rustcorp.com.au>
Date:	Thu, 29 May 2014 20:38:33 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Minchan Kim <minchan@...nel.org>
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

Minchan Kim <minchan@...nel.org> writes:
> 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.

More interesting would be updating your compiler to 4.8, I think.
Saving <100 bytes on virtio is not going to save you, right?

Cheers,
Rusty.
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