Re: Jiffies wraparound is not treated in the schedstats

[Oleg Verych <olecom@flower.upol.cz>]

On 2006-11-09, Balbir Singh <balbir@in.ibm.com> wrote:
> Path: news.gmane.org!not-for-mail
> From: Balbir Singh <balbir@in.ibm.com>
> Newsgroups: gmane.linux.kernel
> Subject: Re: Jiffies wraparound is not treated in the schedstats
> Date: Thu, 09 Nov 2006 11:59:45 +0530
> Organization: IBM
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> Original-To: Mauricio Lin <mauriciolin@gmail.com>
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> Mauricio Lin wrote:
>> Hi Balbir,
>> 
>> Do you know why in the sched_info_arrive() and sched_info_depart()
>> functions the calculation of delta_jiffies does not use the time_after
>> or time_before macro to prevent  the miscalculation when jiffies
>> overflow?
>> 
>> For instance the delta_jiffies variable is simply calculated as:
>> 
>> delta_jiffies = now - t->sched_info.last_queued;
>> 
>> Do not you think the more logical way should be
>> 
>> if (time_after(now, t->sched_info.last_queued))
>>    delta_jiffies = now - t->sched_info.last_queued;
>> else
>>    delta_jiffies = (MAX_JIFFIES - t->sched_info.last_queued) + now
>> 
>
> What's MAX_JIFFIES? Is it MAX_ULONG? jiffies is unsigned long
> so you'll have to be careful with unsigned long arithmetic.
>
> Consider that now is 5 and t->sched_info.last_queued is 10.
>
> On my system
>
> perl -e '{printf("%lu\n", -5 + (1<<32) - 1);}'
> 4294967291

So, according to this
> perl -e '{printf("%lu\n", -5 );}'
> 4294967291
you have 32bit perl (and OS).

I have same result in
"This is perl, v5.8.4 built for i386-linux-thread-multi"

(knoppix 3.2.3 in qemu on intel's amd64).

That means we are on the same question i asked before about integer
overflow. In this case "(1<<32) = 1",  

>> I have included more variables to measure some issues of schedule in
>> the kernel (following schedstat idea) and I noticed that jiffies
>> wraparound has led to wrong values, since the user space tool when
>> collecting the values is producing negative values.
>> 
>
> hmm.. jiffies wrapped around in sched_info_depart()? I've never seen
> that happen. Could you post the additions and user space tool to look at?
> What additional features are you planning to measure in the scheduler?
>
>> Any comments?
>> 
>> Can I provide a patch for that?
>> 
>
> Please feel free to provide patches, this is open source!!
>
>> BR,
>> 
>> Mauricio Lin.
>
>
> -- 
>
> 	Balbir Singh,
> 	Linux Technology Center,
> 	IBM Software Labs

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