linux-kernel - 答复: [????] Re: [????] Re: [????] Re: divide error in x86 and cputime

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Message-ID: <2ebb70da82a047bfbbbdfae4e0e30e48@baidu.com>
Date: Tue, 8 Jul 2025 11:12:02 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: David Laight <david.laight.linux@...il.com>, Steven Rostedt
	<rostedt@...dmis.org>
CC: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"vschneid@...hat.com" <vschneid@...hat.com>, "mgorman@...e.de"
	<mgorman@...e.de>, "bsegall@...gle.com" <bsegall@...gle.com>,
	"dietmar.eggemann@....com" <dietmar.eggemann@....com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"juri.lelli@...hat.com" <juri.lelli@...hat.com>, "mingo@...hat.com"
	<mingo@...hat.com>
Subject: 答复: [????] Re: [????] Re: [????] Re: divide error in x86 and cputime

> > On Tue, 8 Jul 2025 00:10:54 +0000
> > "Li,Rongqing" <lirongqing@...du.com> wrote:
> >
> > > >         stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
> > > > +       /*
> > > > +        * Because mul_u64_u64_div_u64() can approximate on some
> > > > +        * achitectures; enforce the constraint that: a*b/(b+c) <= a.
> > > > +        */
> > > > +       if (unlikely(stime > rtime))
> > > > +               stime = rtime;
> > >
> > >
> > > My 5.10 has not this patch " sched/cputime: Fix
> > > mul_u64_u64_div_u64() precision for cputime ", but I am sure this
> > > patch can not fix this overflow issue, Since division error happened
> > > in mul_u64_u64_div_u64()
> >
> > Have you tried it? Or are you just making an assumption?
> >
> > How can you be so sure? Did you even *look* at the commit?
> 
> It can't be relevant.
> That change is after the mul_u64_u64_div_u64() call that trapped.
> It is also not relevant for x86-64 because it uses the asm version.
> 
> At some point mul_u64_u64_div_u64() got changed to be accurate (and slow) so
> that check isn't needed any more.
> 

I see this patch not relevant

Thank you very much for your confirmation

-Li