Messages in this thread | | | Date | Thu, 20 Jun 2013 01:41:46 -0700 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 6/8] sched: task_sched_runtime introduce micro optimization |
| |
(6/18/13 10:18 AM), Frederic Weisbecker wrote: > On Tue, Jun 18, 2013 at 11:17:41AM -0400, KOSAKI Motohiro wrote: >>>> +#ifdef CONFIG_64BIT >>>> + /* >>>> + * 64-bit doesn't need locks to atomically read a 64bit value. So we >>>> + * have two optimization chances, 1) when caller doesn't need >>>> + * delta_exec and 2) when the task's delta_exec is 0. The former is >>>> + * obvious. The latter is complicated. reading ->on_cpu is racy, but >>>> + * this is ok. If we race with it leaving cpu, we'll take a lock. So >>>> + * we're correct. If we race with it entering cpu, unaccounted time >>>> + * is 0. This is indistinguishable from the read occurring a few >>>> + * cycles earlier. >>>> + */ >>>> + if (!add_delta || !p->on_cpu) >>>> + return p->se.sum_exec_runtime; >>> >>> I'm not sure this is correct from an smp ordering POV. p->on_cpu may appear >>> to be 0 whereas the task is actually running for a while and p->se.sum_exec_runtime >>> can then be past the actual value on the remote CPU. >> >> Quate form Paul's last e-mail >> >>> Stronger: >>> >>> +#ifdef CONFIG_64BIT >>> + if (!p->on_cpu) >>> + return p->se.sum_exec_runtime; >>> +#endif >>> >>> [ Or !p->on_cpu || !add_delta ]. >>> >>> We can take the racy read versus p->on_cpu since: >>> If we race with it leaving cpu: we take lock, we're correct >>> If we race with it entering cpu: unaccounted time ---> 0, this is >>> indistinguishable from the read occurring a few cycles earlier. > > Yeah, my worry was more about both p->on_cpu and p->se.sum_exec_runtime being > stale for too long. How much time can happen in the worst case before CPU X sees > the updates done by a CPU Y under rq(Y)->lock considering that CPU X doesn't take rq(Y) > to read that update? I guess it depends on the hardware, locking and ordering > that happened before.
Right. The worst case depend on memory access cost on remote node. If memory access cost is m, The worst scenario is:
t ------------- 0: CPU X start to fetch p->on_cpu. (now it's 0) 0: CPU Y changes p->on_cpu to 1. m: CPU X sees p->on_cpu is 0.
Then CPU X uses p->se.sum_exec_runtime even though p has delta m. And, in worst case, all cpus make the same scenario. So, inaccuracy should be m x nr_online_cpus. In this case, we can ignore cpu cache because we don't hit anyway in worst case.
However, I think it's ok. m is us order and our run_posx_cpu_timer() only has 1/HZ accuracy. Moreover,taking lock needs more time than m because it need lock prefixed op. Then, It's free lunch.
I have no seen any ordering issue in this code because each CPU have independent time and no dependency. Please tell me if I'm missing something.
> Bah it probably doesn't matter in practice.
I'm ok to drop this "!p->on_cpu" optimization if you really dislike it. It is just a micro optimization and the benefit is not so much.
| |