linux-kernel - Re: [PATCH 4/4] virtio_ring: unify direct/indirect code paths.

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Message-ID: <20140529075256.GZ30445@twins.programming.kicks-ass.net>
Date:	Thu, 29 May 2014 09:52:56 +0200
From:	Peter Zijlstra <peterz@...radead.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>, 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>, 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: [PATCH 4/4] virtio_ring: unify direct/indirect code paths.

On Thu, May 29, 2014 at 04:56:45PM +0930, Rusty Russell wrote:
> virtqueue_add() populates the virtqueue descriptor table from the sgs
> given.  If it uses an indirect descriptor table, then it puts a single
> descriptor in the descriptor table pointing to the kmalloc'ed indirect
> table where the sg is populated.
> 
> Previously vring_add_indirect() did the allocation and the simple
> linear layout.  We replace that with alloc_indirect() which allocates
> the indirect table then chains it like the normal descriptor table so
> we can reuse the core logic.
> 
> Before:
> 	gcc 4.8.2: virtio_blk: stack used = 392
> 	gcc 4.6.4: virtio_blk: stack used = 480
> 
> After:
> 	gcc 4.8.2: virtio_blk: stack used = 408
> 	gcc 4.6.4: virtio_blk: stack used = 432

Is it worth it to make the good compiler worse? People are going to use
the newer GCC more as time goes on anyhow.

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