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

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Date:	Fri, 30 May 2014 08:45:22 +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

On Thu, May 29, 2014 at 08:38:33PM +0930, Rusty Russell wrote:
> 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?

But in my report, virtio_ring consumes more than yours.
As I mentioned other thread to Steven, I don't know why stacktrace report
vring_add_indirect consumes 376-byte. Apparently, objdump says it didn't
consume too much so I'd like to test your patches and see the result.

Thanks.

[ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  10)     6376     376   vring_add_indirect+0x36/0x200
[ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  11)     6000     144   virtqueue_add_sgs+0x2e2/0x320
[ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  12)     5856     288   __virtblk_add_req+0xda/0x1b0
[ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  13)     5568      96   virtio_queue_rq+0xd3/0x1d0

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