linux-kernel - Re: [PATCH v1 1/3] sched/debug: use int type and fix wrong print for rq->nr

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Message-ID: <20230425124856.GE1335080@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Apr 2023 14:48:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     晏艳(采苓) <yanyan.yan@...group.com>
Cc:     linux-kernel@...r.kernel.org,
        谈鉴锋 <henry.tjf@...group.com>,
        Tiwei Bie <tiwei.btw@...group.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v1 1/3] sched/debug: use int type and fix wrong print for
 rq->nr_uninterruptible

On Wed, Apr 12, 2023 at 11:07:29AM +0800, 晏艳(采苓) wrote:
> commit e6fe3f422be1 ("sched: Make multiple runqueue task counters
> 32-bit") changed rq->nr_uninterruptible from 'unsigned long'
> to 'unsigned int', but left the wrong print to
> /sys/kernel/debug/sched/debug and to the console.
> 
> For example:
> Current type is 'unsigned int' and value is fffffff7, and the print
> will run the sentences,
> "do {                                    \
>     if (sizeof(rq->x) == 4)                     \
>         SEQ_printf(m, "  .%-30s: %ld\n", #x, (long)(rq->x));    \

Arguably this is wrong, because LP64 has sizeof(long) != 4. This ought
to be something like:

+               SEQ_printf(m, "  .%-30s: %d\n", #x, (int)(rq->x));      \

>     else                                \
>         SEQ_printf(m, "  .%-30s: %Ld\n", #x, (long long)(rq->x));\
> } while (0)"
> 
> The result will be 4294967287 on 64-bit machines to print (long)(rq->x)
> while old type 'unsigned long' will print -9.
> 
> And the other places that use its value will cast to int to return
> expected output, so we convert its type from 'unsigned int' to int.
> 
> Signed-off-by: Yan Yan <yanyan.yan@...group.com>
> ---
>  kernel/sched/loadavg.c | 2 +-
>  kernel/sched/sched.h   | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index 52c8f8226b0d..b9867495fe8b 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -80,7 +80,7 @@ long calc_load_fold_active(struct rq *this_rq, long adjust)
>  	long nr_active, delta = 0;
>  
>  	nr_active = this_rq->nr_running - adjust;
> -	nr_active += (int)this_rq->nr_uninterruptible;
> +	nr_active += this_rq->nr_uninterruptible;
>  
>  	if (nr_active != this_rq->calc_load_active) {
>  		delta = nr_active - this_rq->calc_load_active;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 060616944d7a..23c643948331 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1006,7 +1006,7 @@ struct rq {
>  	 * one CPU and if it got migrated afterwards it may decrease
>  	 * it on another CPU. Always updated under the runqueue lock:
>  	 */
> -	unsigned int		nr_uninterruptible;
> +	int			nr_uninterruptible;

Bit meh on this; but I suppose since calc_load_fold_active() already had
an explicit cast, it won't make the 'but signed overflow is UB' crowd
more unhappy.