netdev - Re: [PATCH net-next 1/4] unroll: add generic loop unroll helpers

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Message-ID: <Z6nw+s8nPw7JoVeb@boxer>
Date: Mon, 10 Feb 2025 13:28:42 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Simon Horman <horms@...nel.org>
CC: Alexander Lobakin <aleksander.lobakin@...el.com>, Andrew Lunn
	<andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, "Eric
 Dumazet" <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, "Andrii
 Nakryiko" <andrii@...nel.org>, "Jose E. Marchesi" <jose.marchesi@...cle.com>,
	Toke Høiland-Jørgensen <toke@...hat.com>, "Magnus
 Karlsson" <magnus.karlsson@...el.com>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, Jason Baron <jbaron@...mai.com>, "Casey
 Schaufler" <casey@...aufler-ca.com>, Nathan Chancellor <nathan@...nel.org>,
	<bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/4] unroll: add generic loop unroll helpers

On Sun, Feb 09, 2025 at 11:07:25AM +0000, Simon Horman wrote:
> On Thu, Feb 06, 2025 at 07:26:26PM +0100, Alexander Lobakin wrote:
> > There are cases when we need to explicitly unroll loops. For example,
> > cache operations, filling DMA descriptors on very high speeds etc.
> > Add compiler-specific attribute macros to give the compiler a hint
> > that we'd like to unroll a loop.
> > Example usage:
> > 
> >  #define UNROLL_BATCH 8
> > 
> > 	unrolled_count(UNROLL_BATCH)
> > 	for (u32 i = 0; i < UNROLL_BATCH; i++)
> > 		op(priv, i);
> > 
> > Note that sometimes the compilers won't unroll loops if they think this
> > would have worse optimization and perf than without unrolling, and that
> > unroll attributes are available only starting GCC 8. For older compiler
> > versions, no hints/attributes will be applied.
> > For better unrolling/parallelization, don't have any variables that
> > interfere between iterations except for the iterator itself.
> > 
> > Co-developed-by: Jose E. Marchesi <jose.marchesi@...cle.com> # pragmas
> > Signed-off-by: Jose E. Marchesi <jose.marchesi@...cle.com>
> > Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> > Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> 
> Hi Alexander,
> 
> This patch adds four variants of the unrolled helper.  But as far as I can
> tell the patch-set only makes use of one of them, unrolled_count().
> 
> I think it would be best if this patch only added helpers that are used.

That is debatable but I think I tend to agree here. If we add say 3 unused
macros then nothing stops someone from coming up with a patch that deletes
them because they are unused, right?

> 
> ...