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SubjectRe: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.
On 04/23/2012 02:22 PM, Peter Zijlstra wrote:
> On Mon, 2012-04-23 at 14:13 +0200, Juri Lelli wrote:
>> On 04/23/2012 01:32 PM, Peter Zijlstra wrote:
>>> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
>>>> + /*
>>>> + * We Keep moving the deadline away until we get some
>>>> + * available runtime for the entity. This ensures correct
>>>> + * handling of situations where the runtime overrun is
>>>> + * arbitrary large.
>>>> + */
>>>> + while (dl_se->runtime<= 0) {
>>>> + dl_se->deadline += dl_se->dl_deadline;
>>>> + dl_se->runtime += dl_se->dl_runtime;
>>>> + }
>>>
>>> Does gcc 'optimize' that into a division? If so, it might need special
>>> glue to make it not do that.
>>
>> I got two adds and a jle, no div here..
>
> Gcc is known to change such loops to something like:
>
> if (runtime<= 0) {
> tmp = 1 - runtime / dl_runtime;
> deadline += tmp * dl_deadline;
> runtime += tmp * dl_runtime;
> }
>
>

This is what I got for that snippet:

ffffffff81062826 <enqueue_task_dl>:
[...]
ffffffff81062885: 49 03 44 24 20 add 0x20(%r12),%rax
ffffffff8106288a: 49 8b 54 24 28 mov 0x28(%r12),%rdx
ffffffff8106288f: 49 01 54 24 38 add %rdx,0x38(%r12)
ffffffff81062894: 49 89 44 24 30 mov %rax,0x30(%r12)
ffffffff81062899: 49 8b 44 24 30 mov 0x30(%r12),%rax
ffffffff8106289e: 48 85 c0 test %rax,%rax
ffffffff810628a1: 7e e2 jle ffffffff81062885 <enqueue_task_dl+0x5f>

So it seems we are fine in this case, right?
It is anyway better to enforce this Gcc behaviour, just to be
on the safe side?

Thanks,

- Juri


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