Messages in this thread | | | Subject | Re: [PATCH] timers: fix offset calculation when the expires align with LVL_GRAN | From | Xu YiPing <> | Date | Tue, 31 Jul 2018 21:45:11 +0800 |
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On 2018/7/30 19:03, Thomas Gleixner wrote: > 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 <xuyiping@hisilicon.com> >> --- >> 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
LVL_GRAN(0) = 1 and LVL_SHIFT(0) = 0
after the calculation, expires = 1 ?
> 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 > > . >
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