linux-kernel - Re: [PATCH] Revert "timers: Don't wake ktimersoftd on every tick"

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Date:   Mon, 17 Jul 2017 16:58:42 -0500
From:   Haris Okanovic <haris.okanovic@...com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        julia.cartwright@...com, gratian.crisan@...com, harisokn@...il.com
Subject: Re: [PATCH] Revert "timers: Don't wake ktimersoftd on every tick"

On 06/04/2017 09:17 AM, Thomas Gleixner wrote:
> On Fri, 2 Jun 2017, Haris Okanovic wrote:
>> On 05/26/2017 03:50 PM, Thomas Gleixner wrote:
>>>>>   static void expire_timers(struct timer_base *base)
>>>>>   {
>>>>>          struct hlist_head *head;
>>>>> +       int expCount = base->expired_count;
>>>
>>> No camel case for heavens sake!
>>>
>>> And this requires:
>>>
>>>     	 cnt = READ_ONCE(base->expired_count);
>>>
>>>>> -       while (base->expired_count--) {
>>>>> -               head = base->expired_lists + base->expired_count;
>>>>> +       while (expCount--) {
>>>>> +               head = base->expired_lists + expCount;
>>>>>                  __expire_timers(base, head);
>>>>>          }
>>>
>>> Plus a comment.
>>
>> Fixed, thanks.
>>
>> Are your recommending READ_ONCE() purely for documentation purposes?
> 
> Yes.
> 
>>> The other thing I noticed was this weird condition which does not do the
>>> look ahead when base->clk is back for some time.
>>
>> The soft interrupt fires unconditionally if base->clk hasn't advanced in some
>> time to limit how long cpu spends in hard interrupt context.
> 
> That makes no sense.
> 

I wrote this part out of an abundance of caution: I didn't want a 
potentially unbounded operation to run in hardirq context. This logic 
forces both the update to timer bases & firing of timers into softirq 
context if the clock advances by a lot (some arbitrary large number of 
ticks, HZ in this case).

However, I think you're right that this is unneeded since 
run_local_timers() is called per tick, and thus would never exercise 
this case.

Removed this case.

>>> Why don't you use the
>>> existing optimization which uses the bitmap for fast forward?
>>>
>>
>> Are you referring to forward_timer_base()/base->next_expiry? I think it's only
>> updated in the nohz case. Can you share function name/line number(s) if you're
>> thinking of something else.
> 
> I think just using collect_expired_timers() should be enough. In the !NOHZ
> case the base shouldn't be that far back, right?
> 

Refactored.

>>> The other issue I have is that this can race at all. If you raised the
>>> softirq in the look ahead then you should not go into that function until
>>> the softirq has actually completed. There is no point in wasting time in
>>> the hrtimer interrupt if the softirq is running anyway.
>>>
>>
>> Makes sense. Skipping the large `if` block in run_local_timers() when
>> `local_softirq_pending() & TIMER_SOFTIRQ`.
> 
> No. You need your own state tracking. The TIMER_SOFTIRQ bit is cleared when
> the softirq is invoked, but that does not mean that it finished running.
> 
> run_local_timers()
> {
> 	lock(base->lock);
> 	if (!base->softirq_activated)
> 		if (base_has_timers_to_expire()) {
> 			base->softirq_activated = true;
> 			raise_softirq(TIMER_SOFTIRQ);
> 		}
> 	}
> 	unlock(base->lock);
> }
> 
> timer_softirq()
> {
> 	lock(base->lock);
> 	expire_timers();
> 	base->softirq_activated = false;
> 	unlock(base->lock);
> }
> 
> That way you avoid any operation in the tick interrupt as long as the soft
> interrupt processing has not completed.

Added a per-cpu block_softirq boolean.

> 
> Thanks,
> 
> 	tglx
> 

I'll post a v2 patch shortly.

Thanks,
Haris