linux-kernel - Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

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Message-ID: <4A983DA0.9000507@codeaurora.org>
Date:	Fri, 28 Aug 2009 16:27:12 -0400
From:	Ashwin Chaugule <ashwinc@...eaurora.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

Thomas Gleixner wrote:
> On Fri, 28 Aug 2009, Ashwin Chaugule wrote:
>   
>> Thomas Gleixner wrote:
>> Avg startup time 26.4 (10 runs) same as last run.
>>
>> total_calls is again equal to cache_hits ... Its been a while since I wrote my
>> patch. I'll need to look deeper to see if I've done something more. B)
>> Just to make sure, I ran my patch again, I clearly see cache_hits is always
>> less than total_calls.
>>     
>
> Hmm. I'd really like to know why that's behaving different. 
>
> Usually there are only timers in the CLOCK_MONOTONIC base during
> boot. CLOCK_REALTIME base should be empty most of the time. If my
> theory is correct then the number of reprogram events is correct as
> well because base[MONOTONIC]->first is always the one which armed the
> timer.
>
> Can you add some debug which tells us to which base the removed timer
> belongs ?
>   

You're right about that. I confirmed that its MONOTONIC all the way. (I didn't think it'd be like that)
However, the reason for different results, *I think* is coming from hrtimer_reprogram,
where we check if the newly enqueued timer is going to expire before the currently armed timer.
Thats where I change /next_hrtimer/ as well.

So, if the expires_next value is changed as a result of hrtimer_enqueue_reprogram, we just raise HRTIMER_SOFTIRQ (rightfully),
but the latest patch doesn't do those checks in this path, coz, we don't call force_reprogram. 


Cheers,
Ashwin


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