linux-kernel - Re: nohz problem with idle time on old hardware

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140409103138.29be16b9@gandalf.local.home>
Date:	Wed, 9 Apr 2014 10:31:38 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Matthew Whitehead <tedheadster@...il.com>,
	john.stultz@...aro.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mwhitehe@...hat.com
Subject: Re: nohz problem with idle time on old hardware

On Wed, 9 Apr 2014 19:21:53 +0530
Viresh Kumar <viresh.kumar@...aro.org> wrote:

> On Thu, Nov 14, 2013 at 1:31 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > Subject: NOHZ: Check for nohz active instead of nohz enabled
> >
> > RCU and the fine grained idle time accounting functions check
> > tick_nohz_enabled. But that variable is merily telling that NOHZ has
> > been enabled in the config and not been disabled on the command line.
> >
> > But it does not tell anything about nohz being active. That's what all
> > this should check for.
> >
> > Matthew reported, that the idle accounting on his old P1 machine
> > showed bogus values, when he enabled NOHZ in the config and did not
> > disable it on the kernel command line. The reason is that his machine
> > uses (refined) jiffies as a clocksource which explains why the "fine"
> > grained accounting went into lala land, because it depends on when the
> > system goes and leaves idle relative to the jiffies increment.
> >
> > Provide a tick_nohz_active indicator and let RCU and the accounting
> > code use this instead of tick_nohz_enable.
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > @@ -973,7 +968,7 @@ static void tick_nohz_switch_to_nohz(void)
> >         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> >         ktime_t next;
> >
> > -       if (!tick_nohz_enabled)
> > +       if (!tick_nohz_active)
> >                 return;
> 
> Considering the impressive list of Reviewed-by and people involved
> in this patch, I am not sure I am reading the code well here.
> 
> The above change isn't required as per my understanding. Otherwise
> we will never pass that check. tick_nohz_active is initialized as zero
> and so we will keep on returning for ever and wouldn't be able to set
> it to 1 ever.
> 
> I have a patch to fix it up, but wanted to know your opinion before
> sending it.

Ouch! You are correct, this part of the patch makes no sense. That's
what I get for reviewing a patch and not looking at all the code around
the changes. (another kernel developer hangs head in shame :-( )

I think that if statement should be nuked.

-- Steve

> 
> >         local_irq_disable();
> > @@ -981,7 +976,7 @@ static void tick_nohz_switch_to_nohz(void)
> >                 local_irq_enable();
> >                 return;
> >         }
> > -
> > +       tick_nohz_active = 1;
> >         ts->nohz_mode = NOHZ_MODE_LOWRES;
> >
> >         /*

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/