Re: [PATCH] time: tick-sched: use bool for tick_stopped

From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: yuankuiz@codeaurora.org, "Rafael J. Wysocki" <rafael@kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@kernel.org>, Len Brown <len.brown@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-pm-owner@vger.kernel.org
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
Date: Tue, 10 Apr 2018 14:07:32 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1804101407040.2058@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20180410112637.GD4082@hirez.programming.kicks-ass.net>

On Tue, 10 Apr 2018, Peter Zijlstra wrote:

> On Tue, Apr 10, 2018 at 06:07:17PM +0800, yuankuiz@codeaurora.org wrote:
> > > > > > @@ -48,8 +48,8 @@ struct tick_sched {
> > > > > >         unsigned long                   check_clocks;
> > > > > >         enum tick_nohz_mode             nohz_mode;
> > > > > >
> > > > > > +       bool                            tick_stopped    : 1;
> > > > > >         unsigned int                    inidle          : 1;
> > > > > > -       unsigned int                    tick_stopped    : 1;
> > > > > >         unsigned int                    idle_active     : 1;
> > > > > >         unsigned int                    do_timer_last   : 1;
> > > > > >         unsigned int                    got_idle_tick   : 1;
> > > > >
> > > > > I don't think this is a good idea at all.
> > > > >
> > > > > Please see https://lkml.org/lkml/2017/11/21/384 for example.
> > > > [ZJ] Thanks for this sharing. Looks like, this patch fall into the
> > > > case of
> > > > "Maybe".
> > > 
> > > This patch falls into the case 'pointless' because it adds extra storage
> > [ZJ] 1 bit vs 1 bit. no more.
> 
> Since its a different type, the bitfields will not be merged. Also I'm
> surprised a bitfield with base-type _Bool is even allowed.
> 
> > > for no benefit at all.
> > [ZJ] tick_stopped is returned by the tick_nohz_tick_stopped() which is bool.
> > The benefit is no any potiential type conversion could be minded.
> 
> Do you have any actual evidence for that? Is there a compiler stupid
> enough to generate code to convert a bool to a 1bit value?

Sure, if you do:

> > > > > > +       bool                            tick_stopped    : 1;

which is stupidly allowed by the standard....