linux-kernel - Re: [PATCH] kernel: timer: looping issue, need reset variable 'found'

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Date:	Tue, 18 Jun 2013 18:42:48 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel: timer: looping issue, need reset variable 'found'

On 06/10/2013 10:12 PM, Thomas Gleixner wrote:
> On Sun, 9 Jun 2013, Chen Gang wrote:
> 
>>
>> According to __internal_add_timer(), in _next_timer_interrupt(), when
>> 'tv1.vec' find one, but need 'cascade bucket(s)', we still need find
>> each slot of 'tv*.vec'.
> 
> No, we do not. We only need to scan the first cascade array after the
> enqueued timer. If there is nothing in tv2 which might come before the
> found timer, then any timer in tv3 will be later than the one we found
> in the primary wheel.
> 

If we assume "If there is nothing in tv2 which might come before the
found timer, then any timer in tv3 will ..." is correct.

When we found a timer in 'tv1', we will not search all timers in 'tv2'
(we only search first looping of tv2 for the specific 'slot').

Is it still OK ?

>> So need reset variable 'found', so can fully scan ''do {...} while()''
>> for 'tv*.vec'.
> 
> And thereby lose the information, that we already found a timer in the
> scan of the primary array.
> 

When we found a timer, 'expires' would be set.  So resetting 'found' is
still correct, but may let performance lower (if original implement is
correct too)

I think we can treat original implementation as for speed optimization,
so our discussion is "whether this speed optimization has effect with
correctness".

Thanks.
-- 
Chen Gang

Asianux Corporation
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