On Tue, Jun 18, 2013 at 11:17 AM, KOSAKI Motohiro <[email protected]> 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. > > That said, even though we got slightly inaccurate current time, we > have no way to > know this is inaccurate. E.g. cpu clock saving feature bring us more > inaccuracy, but > we already live in such world.
Ah, RT folks may want to call preempt_disable() in thread_group_cputime() because preemptive kernel can be preemptible while for-each-threads loop for getting accurate time. But it is another story, it's not new issue and it's not introduced by me. :-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
