On Fri, 27 Jul 2018, Xu YiPing wrote: > when the expires of timer is align with LVL_GRAN(n), it will be trigged > in 'expires + LVL_GRAN(n)'. > > Some drivers like power runtime use the timer to start a power down > of device, it could saves power if the timer is triggerd in time, > especially when LEVEL=0 with low expires.
>From the above I have no idea what you are trying to 'fix', but see below. > Signed-off-by: Xu YiPing <[email protected]> > --- > kernel/time/timer.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/time/timer.c b/kernel/time/timer.c > index cc2d23e..76655e2 100644 > --- a/kernel/time/timer.c > +++ b/kernel/time/timer.c > @@ -487,7 +487,8 @@ 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 = (expires + LVL_GRAN(lvl) - 1) >> LVL_SHIFT(lvl); > + > return LVL_OFFS(lvl) + (expires & LVL_MASK); This is fundamentally wrong. Assume the following scenario: base->clk = 1; timer->expires = 1; __internal_add_timer(base, timer) { idx = calc_wheel_index(timer->expires, base->clk) { delta = expires - clk; if (delta < LVL_START(1)) idx = calc_index(expires, 0) { expires = (expires + LVL_GRAN(0) - 1) >> LVL_SHIFT(0); return LVL_OFFS(0) + (expires & LVL_MASK); Now lets use real numbers: __internal_add_timer(base, timer) { idx = calc_wheel_index(1, 1) { delta = 1 - 1; <- 0 if (0 < LVL_START(1)) idx = calc_index(1, 0) { expires = (1 + LVL_GRAN(0) - 1) >> LVL_SHIFT(0); ----> expires = 0 return LVL_OFFS(0) + (0 & LVL_MASK); ----> 0 So the returned index is 0, which means that the timer will expire in LVL_SIZE - 1 == 63 ticks. The above example is the worst case, but you broke other assumptions as well. The timer wheel guarantees that a timer armed with: mod_timer(timer, jiffies + 1) will not fire before aty least one jiffy has elapsed. Let's look at the time line: |-------------------|-------------------|----------------| tick tick tick jiffies jiffies + 1 jiffies + 2 | | | Any timer armed | ^ | here must be | | | queued here -------------------------| in order to guarantee that. Timer wheel timers are not accurate and never can be. Thanks, tglx
