On Tue, 10 Apr 2018, Peter Zijlstra wrote:
> On Tue, Apr 10, 2018 at 06:07:17PM +0800, yuank...@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....
