linux-kernel - Re: [PATCH 21/23] y2038: itimer: change implementation to timespec64

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Message-ID: <alpine.DEB.2.21.1911141147220.2507@nanos.tec.linutronix.de>
Date:   Thu, 14 Nov 2019 11:48:08 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     Arnd Bergmann <arnd@...db.de>, y2038@...ts.linaro.org,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 21/23] y2038: itimer: change implementation to
 timespec64

On Wed, 13 Nov 2019, Steven Rostedt wrote:
> On Wed, 13 Nov 2019 23:28:47 +0100 (CET)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Fri, 8 Nov 2019, Arnd Bergmann wrote:
> > >  TRACE_EVENT(itimer_state,
> > >  
> > > -	TP_PROTO(int which, const struct itimerval *const value,
> > > +	TP_PROTO(int which, const struct itimerspec64 *const value,
> > >  		 unsigned long long expires),
> > >  
> > >  	TP_ARGS(which, value, expires),
> > > @@ -321,12 +321,12 @@ TRACE_EVENT(itimer_state,
> > >  		__entry->which		= which;
> > >  		__entry->expires	= expires;
> > >  		__entry->value_sec	= value->it_value.tv_sec;
> > > -		__entry->value_usec	= value->it_value.tv_usec;
> > > +		__entry->value_usec	= value->it_value.tv_nsec / NSEC_PER_USEC;
> > >  		__entry->interval_sec	= value->it_interval.tv_sec;
> > > -		__entry->interval_usec	= value->it_interval.tv_usec;
> > > +		__entry->interval_usec	= value->it_interval.tv_nsec / NSEC_PER_USEC;  
> > 
> > Hmm, having a division in a tracepoint is clearly suboptimal.
> 
> Right, we should move the division into the TP_printk()
> 
> 		__entry->interval_nsec = alue->it_interval.tv_nsec;
> 
> > 
> > >  	),
> > >  
> > > -	TP_printk("which=%d expires=%llu it_value=%ld.%ld it_interval=%ld.%ld",
> > > +	TP_printk("which=%d expires=%llu it_value=%ld.%06ld it_interval=%ld.%06ld",  
> > 
> > We print only 6 digits after the . so that would be even correct w/o a
> > division. But it probably does not matter much.
> 
> Well, we still need the division in the printk, otherwise it will print
> more than 6. That's just the minimum and it will print the full number.

That's fine. The print is not really timing critical, the tracepoint very
much so.

Thanks,

	tglx