site_archiver@lists.apple.com Delivered-To: darwin-kernel@lists.apple.com -- Terry _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.a... On Sep 7, 2007, at 1:08 AM, Anton Altaparmakov wrote: On 7 Sep 2007, at 03:55, Michael Smith wrote: On Sep 6, 2007, at 5:03 PM, Régis Duchesne wrote: Is there a way to make the current thread yield the processor in a kext? Not in the fashion you describe. If you have work, do it. If the scheduler has more important work that has to be done, it will take the CPU away from your thread. But for example from a file system kext we don't know what priority the thread is and we don't own the thread either so we should not be messing with priorities... And what if we have work to do but can't do it because we need another thread to do work first? It is stupid to hang onto our time slice in at this point... You block on the mutex held by the other thread that you know you need to wait for because it holds the mutex. Then the scheduler does it's normal priority lending trick, and you implicitly yield anyway, and the other thread gets the priority boost necessary to get its work done ASAP and get you unblocked ASAP. The only reason to want to do this is if you are performing a long- running low priority task on the wrong (high-priority) thread. Don't do that. Do the work on a thread whose priority reflects the true priority of the task. It's not your business to make scheduling decisions; don't even try. Your view is too limited. This is not true at all. I will give you a concrete example of when I would like to yield the CPU from my file system kext: I need to take a lock but I can't do it safely because of holding other locks so I do a try-lock. The try-lock fails so I have to drop my locks and try again. It is totally brain damaged to try again immediately. It would be far more sensible to yield the CPU to give the other process holding my lock to finish its work and release it and when the scheduler next gets to my process for me to try again and if I fail again to yield again, etc... And trying again immediately is actually MUCH worse than the fact that CPU is just being burned unnecessarily because the other process might be trying to take the lock I just dropped so if I take it again immediately the chances of the other process taking that lock successfully and hence making progress thus releasing both locks so my process can make progress are close to zero at this point! )-: Except that the scheduler is going to raise your priority until it eclipses everything else because you constantly fail to utilize your full quantum, yet are constantly runnable. I expect that you would be unable to actually rool your state back forward immediately, though, since, having backed out your state, the other thread is able to make progress, and you won't be able to immediately acquire your mutex any more. But assuming instead that you aren't trying to fight a lock order inversion with a scheduler hack here, the answer would be "call IOSleep or consider refactoring your code so that this doesn't become an issue in the first place". Like maybe use a deadicated worker thread and an IOWorkQueue. And another example: In a critical path when a memory allocation fails, I want to yield the CPU and try again later when the system hopefully has more memory available as failing the allocation is a disaster for file system consistency at this point. At the moment I just do it in a busy loop trying again and again and again till the scheduler decides to reschedule me. That's got to be totally insane... )-: Set the M_WAIT flag so the allocation blocks until it can be satisfied or the heat death of the universe, whichever comes first. I would very much like to know how I can tell the scheduler "I am done, hand CPU to someone else now"... Generally, by blocking on something that, when there is work for you to do, will wake you up (or by returning to your caller, if you are on a user space thread context instead of your own worker thread). The kernel either lets you run, preempts you, because you are interruptible, or lets you block waiting for a resource ("more work to do" is a resource). This email sent to site_archiver@lists.apple.com