linux-kernel - Re: [PATCH] sched/cpuacct: Fix charge cpuacct.usage

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Message-ID: <20200416120132.7c2df52a@gandalf.local.home>
Date:   Thu, 16 Apr 2020 12:01:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, mingo@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/cpuacct: Fix charge cpuacct.usage_sys
 incorrently.

On Thu, 16 Apr 2020 11:35:02 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 16 Apr 2020 22:18:33 +0800
> Muchun Song <songmuchun@...edance.com> wrote:
> 
> > The user_mode(task_pt_regs(tsk)) always return true for
> > user thread, and false for kernel thread. So it means that
> > the cpuacct.usage_sys is the time that kernel thread uses
> > not the time that thread uses in the kernel mode. We can
> > use get_irq_regs() instead of task_pt_regs() to fix it.
> > 
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > ---
> >  kernel/sched/cpuacct.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
> > index 6448b0438ffb2..edfc62554648e 100644
> > --- a/kernel/sched/cpuacct.c
> > +++ b/kernel/sched/cpuacct.c
> > @@ -5,6 +5,7 @@
> >   * Based on the work by Paul Menage (menage@...gle.com) and Balbir Singh
> >   * (balbir@...ibm.com).
> >   */
> > +#include <asm/irq_regs.h>
> >  #include "sched.h"
> >  
> >  /* Time spent by the tasks of the CPU accounting group executing in ... */
> > @@ -339,7 +340,7 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
> >  {
> >  	struct cpuacct *ca;
> >  	int index = CPUACCT_STAT_SYSTEM;
> > -	struct pt_regs *regs = task_pt_regs(tsk);
> > +	struct pt_regs *regs = get_irq_regs();  
> 
> But get_irq_regs() is only available from interrupt context. This will be
> NULL most the time, whereas the original way will have regs existing for
> the task.

Perhaps you want:

	regs = get_irqs_regs();
	if (!regs)
		regs = task_pt_regs(tsk);

?

-- Steve

> 
> >  
> >  	if (regs && user_mode(regs))
> >  		index = CPUACCT_STAT_USER;  
>