Messages in this thread | | | Date | Mon, 23 Apr 2012 15:37:06 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation. |
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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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