linux-kernel - Re: [PATCH] timers: Fix get_next_timer

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Date:   Mon, 12 Jul 2021 14:04:29 +0800
From:   He Zhe <zhe.he@...driver.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     anna-maria@...utronix.de, linux-kernel@...r.kernel.org,
        tglx@...utronix.de
Subject: Re: [PATCH] timers: Fix get_next_timer_interrupt() with no timers
 pending



On 7/10/21 5:05 PM, Frederic Weisbecker wrote:
> On Fri, Jul 09, 2021 at 04:13:25PM +0200, Nicolas Saenz Julienne wrote:
>> 31cd0e119d50 ("timers: Recalculate next timer interrupt only when
>> necessary") subtly altered get_next_timer_interrupt()'s behaviour. The
>> function no longer consistently returns KTIME_MAX with no timers
>> pending.
>>
>> In order to decide if there are any timers pending we check whether the
>> next expiry will happen NEXT_TIMER_MAX_DELTA jiffies from now.
>> Unfortunately, the next expiry time and the timer base clock are no
>> longer updated in unison. The former changes upon certain timer
>> operations (enqueue, expire, detach), whereas the latter keeps track of
>> jiffies as they move forward. Ultimately breaking the logic above.
>>
>> A simplified example:
>>
>> - Upon entering get_next_timer_interrupt() with:
>>
>> 	jiffies = 1
>> 	base->clk = 0;
>> 	base->next_expiry = NEXT_TIMER_MAX_DELTA;
>>
>>   'base->next_expiry == base->clk + NEXT_TIMER_MAX_DELTA', the function
>>   returns KTIME_MAX.
>>
>> - 'base->clk' is updated to the jiffies value.
>>
>> - The next time we enter get_next_timer_interrupt(), taking into account
>>   no timer operations happened:
>>
>> 	base->clk = 1;
>> 	base->next_expiry = NEXT_TIMER_MAX_DELTA;
>>
>>   'base->next_expiry != base->clk + NEXT_TIMER_MAX_DELTA', the function
>>   returns a valid expire time, which is incorrect.
>>
>> This ultimately might unnecessarily rearm sched's timer on nohz_full
>> setups, and add latency to the system[1].
>>
>> So, introduce 'base->timers_pending'[2], update it every time
>> 'base->next_expiry' changes, and use it in get_next_timer_interrupt().
>>
>> [1] See tick_nohz_stop_tick().
>> [2] A quick pahole check on x86_64 and arm64 shows it doesn't make
>>     'struct timer_base' any bigger.
>>
>> Fixes: 31cd0e119d50 ("timers: Recalculate next timer interrupt only when necessary")
>> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> He Zhe, does it fix your issue?

Yes, this fixes my issue. Thank you all.

Zhe

>
> Thanks.