From: Thomas Gleixner <tglx@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Anna-Maria Gleixner <anna-maria@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index()
Date: Thu, 02 Jul 2020 13:59:17 +0200 [thread overview]
Message-ID: <87pn9e9lgq.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200701011030.14324-4-frederic@kernel.org>
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?
> @@ -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.
Thanks,
tglx
next prev parent reply other threads:[~2020-07-02 11:59 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-01 1:10 [RFC PATCH 00/10] timer: Reduce timers softirq (and other optimizations) Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 01/10] timer: Prevent base->clk from moving backward Frederic Weisbecker
2020-07-01 16:35 ` Juri Lelli
2020-07-01 23:20 ` Frederic Weisbecker
2020-07-02 9:59 ` Juri Lelli
2020-07-02 14:04 ` Frederic Weisbecker
2020-07-02 14:32 ` Frederic Weisbecker
2020-07-02 15:57 ` Juri Lelli
2020-07-02 9:48 ` Thomas Gleixner
2020-07-01 1:10 ` [RFC PATCH 02/10] timer: Move trigger_dyntick_cpu() to enqueue_timer() Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index() Frederic Weisbecker
2020-07-02 11:59 ` Thomas Gleixner [this message]
2020-07-02 12:27 ` Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 04/10] timer: Optimize _next_timer_interrupt() level iteration Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 05/10] timers: Always keep track of next expiry Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 06/10] timer: Reuse next expiry cache after nohz exit Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 07/10] timer: Expand clk forward logic beyond nohz Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 08/10] timer: Spare timer softirq until next expiry Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 09/10] timer: Remove must_forward_clk Frederic Weisbecker
2020-07-01 1:10 ` [RFC PATCH 10/10] timer: Lower base clock forwarding threshold Frederic Weisbecker
2020-07-02 13:21 ` Thomas Gleixner
2020-07-02 13:32 ` Frederic Weisbecker
2020-07-02 15:14 ` Thomas Gleixner
2020-07-03 0:12 ` Frederic Weisbecker
2020-07-03 9:13 ` Thomas Gleixner
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