linux-kernel - Re: [PATCH] cpuidle: idle exit

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Message-ID: <81ad0d7dcb82c6f1f49e6d0882249231682c1b6c.camel@mediatek.com>
Date:   Thu, 16 Nov 2023 14:52:08 +0000
From:   Bo Ye (叶波) <Bo.Ye@...iatek.com>
To:     "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>,
        "rafael@...nel.org" <rafael@...nel.org>
CC:     Yongdong Zhang (张永东) 
        <Yongdong.Zhang@...iatek.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        C Cheng (陈程) <C.Cheng@...iatek.com>
Subject: Re: [PATCH] cpuidle: idle exit_latency overflow

On Thu, 2023-11-16 at 15:20 +0100, AngeloGioacchino Del Regno wrote:
> Il 16/11/23 14:26, Bo Ye ha scritto:
> > From: C Cheng <C.Cheng@...iatek.com>
> > 
> > In detail:
> > In kernel-6.1, in the __cpuidle_driver_init function in
> > driver/cpuidle/driver.c, there is a line of code that causes
> > an overflow. The line is s->exit_latency_ns = s->exit_latency
> > * NSEC_PER_USEC. The overflow occurs because the product of an
> > int type and a constant exceeds the range of the int type.
> > 
> > In C language, when you perform a multiplication operation, if
> > both operands are of int type, the multiplication operation is
> > performed on the int type, and then the result is converted to
> > the target type. This means that if the product of int type
> > multiplication exceeds the range that int type can represent,
> > an overflow will occur even if you store the result in a
> > variable of int64_t type.
> > 
> > Signed-off-by: C Cheng <C.Cheng@...iatek.com>
> 
> It all makes sense, but "mtk24676" is not the name of a person,
> what's going on?
  Corrected, apologies for any inconvenience caused.
> 
> > Signed-off-by: Bo Ye <bo.ye@...iatek.com>
> 
> That's the same, you perhaps wanted to write "Bo Ye", not "bo.ye".
  Corrected, apologies for any inconvenience caused. I would greatly
appreciate it if you could recommend a formatting checking tool.
  If there is no auto checking tool, I will pay attention to these
formatting checks next time.

Best Regards,
Bosser
> 
> Regards,
> Angelo
> 
> > ---
> > 
> > diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> > index d9cda7f..631ca16 100644
> > --- a/drivers/cpuidle/driver.c
> > +++ b/drivers/cpuidle/driver.c
> > @@ -187,7 +187,7 @@
> >   			s->target_residency = div_u64(s-
> > >target_residency_ns, NSEC_PER_USEC);
> >   
> >   		if (s->exit_latency > 0)
> > -			s->exit_latency_ns = s->exit_latency *
> > NSEC_PER_USEC;
> > +			s->exit_latency_ns = (u64)s->exit_latency *
> > NSEC_PER_USEC;
> >   		else if (s->exit_latency_ns < 0)
> >   			s->exit_latency_ns =  0;
> >   		else
> 
>