Messages in this thread | | | Date | Sat, 12 May 2018 13:26:47 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state |
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On Sat, 12 May 2018 07:41:19 -0700 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> Don't get me wrong, this discussion was quite useful to me. We probably > need to at least change the comments, and perhaps the code as well. But > I agree that we need input from Peter and Steven to make much more forward > progress.
It's the weekend so I skimmed more than read this thread, but I will just add this.
The table Joel posted is interesting, and perhaps we should keep things consistent with that. But that said, with respect to task-RCU, as nothing on a trampoline should ever call cond_resched() (and perhaps I should add code in lockdep that verifies this), we just want a quiescent state that tells us that the task has left the trampoline. A cond_resched() should be one of those points that does.
It really has nothing to do with scheduling or preemption. The issue is that if a task is on a trampoline and gets preempted, there's no knowing when it is off that trampoline where we can free it. We need to have places in the kernel that we know is a quiescent state to move task-RCU forward. cond_resched() seems to be one of them. schedule itself can not be, because it can be called from an interrupt preempting a task while it is on the trampoline.
-- Steve
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