linux-kernel - Re: [PATCH 4.14 054/293] sched/fair: Fix "runnable_avg_yN

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Date:   Tue, 30 Jul 2019 11:28:01 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Qian Cai <cai@....pw>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.14 054/293] sched/fair: Fix "runnable_avg_yN_inv" not
 used warnings

On Mon, Jul 29, 2019 at 09:19:05PM +0200, Greg Kroah-Hartman wrote:
> [ Upstream commit 509466b7d480bc5d22e90b9fbe6122ae0e2fbe39 ]
> 
> runnable_avg_yN_inv[] is only used in kernel/sched/pelt.c but was
> included in several other places because they need other macros all
> came from kernel/sched/sched-pelt.h which was generated by
> Documentation/scheduler/sched-pelt. As the result, it causes compilation
> a lot of warnings,
> 
>   kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=]
>   kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=]
>   kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=]
>   ...
> 
> Silence it by appending the __maybe_unused attribute for it, so all
> generated variables and macros can still be kept in the same file.
> 
> Signed-off-by: Qian Cai <cai@....pw>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lkml.kernel.org/r/1559596304-31581-1-git-send-email-cai@lca.pw
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  Documentation/scheduler/sched-pelt.c | 3 ++-
>  kernel/sched/sched-pelt.h            | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-pelt.c b/Documentation/scheduler/sched-pelt.c
> index e4219139386a..7238b355919c 100644
> --- a/Documentation/scheduler/sched-pelt.c
> +++ b/Documentation/scheduler/sched-pelt.c
> @@ -20,7 +20,8 @@ void calc_runnable_avg_yN_inv(void)
>  	int i;
>  	unsigned int x;
>  
> -	printf("static const u32 runnable_avg_yN_inv[] = {");
> +	/* To silence -Wunused-but-set-variable warnings. */
> +	printf("static const u32 runnable_avg_yN_inv[] __maybe_unused = {");
>  	for (i = 0; i < HALFLIFE; i++) {
>  		x = ((1UL<<32)-1)*pow(y, i);
>  
> diff --git a/kernel/sched/sched-pelt.h b/kernel/sched/sched-pelt.h
> index a26473674fb7..c529706bed11 100644
> --- a/kernel/sched/sched-pelt.h
> +++ b/kernel/sched/sched-pelt.h
> @@ -1,7 +1,7 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  /* Generated by Documentation/scheduler/sched-pelt; do not modify. */
>  
> -static const u32 runnable_avg_yN_inv[] = {
> +static const u32 runnable_avg_yN_inv[] __maybe_unused = {
>  	0xffffffff, 0xfa83b2da, 0xf5257d14, 0xefe4b99a, 0xeac0c6e6, 0xe5b906e6,
>  	0xe0ccdeeb, 0xdbfbb796, 0xd744fcc9, 0xd2a81d91, 0xce248c14, 0xc9b9bd85,
>  	0xc5672a10, 0xc12c4cc9, 0xbd08a39e, 0xb8fbaf46, 0xb504f333, 0xb123f581,
> -- 
> 2.20.1
> 
> 
> 

This patch does nothing in 4.14. There is no kernel/sched/pelt.c.

$ fd pelt
kernel/sched/sched-pelt.h
Documentation/scheduler/sched-pelt.c

runnable_avg_yN_inv is only used in one location in 4.14.134, in fair.c
behind CONFIG_SMP, which is also the guard that includes sched-pelt.h.

$ rg --no-heading -g '!Documentation' runnable_avg_yN_inv
kernel/sched/sched-pelt.h:4:static const u32 runnable_avg_yN_inv[] = {
kernel/sched/fair.c:2849:       val = mul_u64_u32_shr(val, runnable_avg_yN_inv[local_n], 32);

$ rg --no-heading -B 2 "sched-pelt.h"
kernel/sched/fair.c-707-#ifdef CONFIG_SMP
kernel/sched/fair.c-708-
kernel/sched/fair.c:709:#include "sched-pelt.h"

I see no way for the warnings in this patch to occur here, making it
pointless. I get two trivial conflicts in my msm-4.14 tree from this
patch that I would like to avoid dealing with.

Please consider dropping this patch, thanks!
Nathan