Re: [RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index()

Frederic Weisbecker
2020-07-02T12:27:06Z

On Thu, Jul 02, 2020 at 01:59:17PM +0200, Thomas Gleixner wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
> > LVL_START() makes the first index of a level to start with what would be
> > the value of all bits set of the previous level.
> >
> > For example level 1 starts at 63 instead of 64.
> >
> > To cope with that, calc_index() always adds one offset for the level
> > granularity to the expiry passed in parameter.
> >
> > Yet there is no apparent reason for such fixups so simplify the whole
> > thing.
>
> You sure?

No :o)

>
> > @@ -158,7 +158,7 @@ EXPORT_SYMBOL(jiffies_64);
> > * The time start value for each level to select the bucket at enqueue
> > * time.
> > */
> > -#define LVL_START(n) ((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
> > +#define LVL_START(n) (LVL_SIZE << (((n) - 1) * LVL_CLK_SHIFT))
>
> > /* Size of each clock level */
> > #define LVL_BITS 6
> > @@ -489,7 +489,7 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
> > */
> > static inline unsigned calc_index(unsigned expires, unsigned lvl)
> > {
> > - expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
> > + expires >>= LVL_SHIFT(lvl);
> > return LVL_OFFS(lvl) + (expires & LVL_MASK);
> > }
>
> So with that you move the expiry of each timer one jiffie ahead vs. the
> original code, which violates the guarantee that a timer sleeps at least
> for one jiffie for real and not measured in jiffies.
>
> base->clk = 1
> jiffies = 0
> local_irq_disable()
> -> timer interrupt is raised in HW
> timer->expires = 1
> add_timer(timer)
> ---> index == 1
> local_irq_enable()
> timer interrupt
> jiffies++;
> softirq()
> expire(timer);
>
> So the off by one has a reason.

Fair enough. I didn't know about the one jiffy sleep guarantee.
I'll convert this patch to a comment explaining that off by one,
using your above example.

Thanks!