Re: [PATCH V2] tick: Make sleep length calculation more accurate

[Daniel Lezcano]

On 10/02/2013 06:42 PM, Frederic Weisbecker wrote:
> On Wed, Oct 02, 2013 at 06:22:29PM +0200, Daniel Lezcano wrote:
> > On 10/02/2013 05:57 PM, Frederic Weisbecker wrote:
> > > 2013/10/2 Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>:
> > > > The sleep_length is computed in the tick_nohz_stop_sched_tick function but it
> > > > is used later in the code with in between the local irq enabled.
> > > >
> > > > cpu_idle_loop
> > > >    tick_nohz_idle_enter         [ exits with local irq enabled ]
> > > >     __tick_nohz_idle_enter
> > > >       tick_nohz_stop_sched_tick
> > > >    ...
> > > >
> > > >    arch_cpu_idle
> > > >       menu_select               [ uses here 'sleep_length' ]
> > > >    ...
> > > >
> > > > Between the computation of the sleep length and its usage, some interrupts
> > > > may occur, making the sleep length shorter than actually it is because of the
> > > > interrupt processing
> > >
> > > So, do you mean that the ts->sleep_length would return a value that is too long
> > > given that the CPU already spent some time to service the irqs since we computed
> > > the sleep length in tick_nohz_idle_enter()?
> > >
> > > But then tick_nohz_irq_exit() should take care of that as it calls
> > > again tick_nohz_stop_sched_tick().
> > > So I'm a bit confused.
> > >
> > > > or different if the timer itself expired.
> > >
> > > Same here, if the timer expired, it triggers an interrupt which can do
> > > two things:
> > >
> > > 1) reprogram a new timer and this recompute sleep_length
> > > 2) set_need_resched() and then exit the idle loop, so arch_cpu_idle() won't even
> > > be called. Or the timer interrupts hlt, but then menu_select() was
> > > called before.
> > >
> > > So I probably missed something here.
> >
> > No you did not :)
> >
> > Indeed... At the first glance, this issue sounded so obvious I
> > suspected there must be a trick somewhere but I did not think to
> > look at the irq_exit, the code is very complex. Thanks for
> > clarifying this.
> >
> > For my personal information, is there any particular reason to set
> > an intermediate 'sleep_length' in tick_nohz_stop_sched_tick instead
> > of doing what does this patch ?
>
> May be we could do it that way yeah. Is menu_select() called only there?
> I don't know how much difference that would make.

Yes, it is called just one time in all the code. The benefit would be 
just to cleanup a field in the struct tick_sched.


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