Re: [PATCH v3] sched/deadline: overrun could happen in start_hrtick_dl

["xiaofeng.yan" <xiaofeng.yan@huawei.com>]

On 2014/8/12 22:52, Ingo Molnar wrote:
> * xiaofeng.yan <xiaofeng.yan@huawei.com> wrote:
>
>> It could be wrong for the precision of runtime and deadline
>> when the precision is within microsecond level. For example:
>> Task runtime deadline period
>>   P1   200us   500us   500us
>>
>> This case need enbale HRTICK feature by the next command
>>
>> PC#echo "HRTICK" > /sys/kernel/debug/sched_features
>> PC#trace-cmd record -e sched_switch &
>> PC#./schedtool -E -t 200000:500000 -e ./test
>>
>> Some of runtime and deadline run with millisecond level by
>> reading kernershark. Some pieces of trace.dat are as follows:
>> (remove some irrelevant information)
>> <idle>-0   157.603157: sched_switch: :R ==> 2481:4294967295: test
>> test-2481  157.603203: sched_switch:  2481:R ==> 0:120: swapper/2
>> <idle>-0   157.605657: sched_switch:  :R ==> 2481:4294967295: test
>> test-2481  157.608183: sched_switch:  2481:R ==> 2483:120: trace-cmd
>> trace-cmd-2483 157.609656: sched_switch:2483:R==>2481:4294967295: test
>>
>> We can get the runtime from the information at some point.
>> runtime = 157.605657 - 157.608183
>> runtime = 0.002526(2.526ms)
>> The correct runtime should be less than or equal to 200us at some point.
>>
>> The problem is caused by a conditional judgment "delta > 10000".
>> Because no hrtimer start up to control the runtime when runtime is less than 10us.
>> So the process will continue to run until tick-period coming.
>>
>> Move the code with the limit of the least time slice
>> from hrtick_start_fair() to hrtick_start() because
>> EDF schedule class also need this function in start_hrtick_dl().
>>
>> To fix this problem, we call hrtimer_start() unconditionally in start_hrtick_dl(),
>> and make sure schedule slice won't be smaller than 10us in hrtimer_start().
>>
>> Signed-off-by: Xiaofeng Yan <xiaofeng.yan@huawei.com>
>> Reviewed-by:   Peter Zijlstra <peterz@infradead.org>
>> Reviewed-by:   Li Zefan <lizefan@huawei.com>
> The whole changelog is very hard to read and isn't proper English, nor
> is it truly explanatory. Could you please fix the changelog, or bounce
> it to someone who will fix it for you?
>
> Thanks,
>
> 	Ingo

Thanks for your reply. I will fix my change log with proper English.
Thanks
Yan
> .
>