Messages in this thread |  | | Date | Thu, 28 Jun 2001 00:41:29 +0200 | From | Manfred Spraul <> | Subject | Re: wake_up vs. wake_up_sync |
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Mike Kravetz wrote: > > On Wed, Jun 27, 2001 at 11:22:19PM +0200, Manfred Spraul wrote: > > > Why would you want to prevent > > > reschedule_idle()? > > > > > If one process runs, wakes up another process and _knows_ that it's > > going to sleep immediately after the wake_up it doesn't need the > > reschedule_idle: the current cpu will be idle soon, the scheduler > > doesn't need to find another cpu for the woken up thread. > > I'm curious. How does the caller of wake_up_sync know that the > current cpu will soon be idle. Does it assume that there are no > other tasks on the runqueue waiting for a CPU? If there are other > tasks on the runqueue, isn't it possible that another task has a > higher goodness value than the task being awakened. In such a case, > isn't is possible that the awakened task could sit on the runqueue > (waiting for a CPU) while tasks with a lower goodness value are > allowed to run? >
I found one combination where that could happen:
process.thread A.1: highest priority, runs on cpu0 B.1: lowest priority, runs on cpu1 A.2: another thread of process A, priority PROC_CHANGE_PENALTY+PRIORITY(B.1)+1, sleeping. B.2: same priority as A.2, sleeping, same process as B.1
A.1: { wake_up("A.2"); /* nothing happens: preemption_goodness is 0 since B.1 has both PROC_CHANGE_PENALTY and the += 1 from 'same mm' */ wake_up_sync("B.2"); schedule(); /* schedule selects A.2 instead of B.2 due to the += 1 from 'same mm'. BUG: B.2 should replace B.1 on cpu1. The preemption_goodness is 1. */
IMHO obscure and very rare.
But I just found a bigger problem: If wake_up_sync wakes up more than 1 process then cpus could remain in cpu_idle() although processes are on the runqueue without cpus.
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