On Tue, May 05, 2015 at 12:51:02PM +0200, Peter Zijlstra wrote: > On Tue, May 05, 2015 at 12:48:34PM +0200, Peter Zijlstra wrote: > > On Mon, May 04, 2015 at 03:00:44PM -0400, Rik van Riel wrote: > > In case of the non-preemptible RCU, we could easily also > > > increase current->rcu_read_lock_nesting at the same time > > > we increase the preempt counter, and use that as the > > > indicator to test whether the cpu is in an extended > > > rcu quiescent state. That way there would be no extra > > > overhead at syscall entry or exit at all. The trick > > > would be getting the preempt count and the rcu read > > > lock nesting count in the same cache line for each task. > > > > Can't do that. Remember, on x86 we have per-cpu preempt count, and your > > rcu_read_lock_nesting is per task. > > Hmm, I suppose you could do the rcu_read_lock_nesting thing in a per-cpu > counter too and transfer that into the task_struct on context switch. > > If you manage to put both sides of that in the same cache things should > not add significant overhead. > > You'd have to move the rcu_read_lock_nesting into the thread_info, which > would be painful as you'd have to go touch all archs etc..
Last I tried doing that, things got really messy at context-switch time. Perhaps I simply didn't do the save/restore in the right place? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/