On Tue, May 05, 2015 at 03:00:26PM +0200, Peter Zijlstra wrote: > On Tue, May 05, 2015 at 05:34:46AM -0700, Paul E. McKenney wrote: > > On Tue, May 05, 2015 at 12:53:46PM +0200, Peter Zijlstra wrote: > > > On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote: > > > > But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt > > > > counter in production kernels. Even if there was, we have to sample > > > > this > > > > on other CPUs, so the overhead of preempt_disable() and preempt_enable() > > > > would be where kernel entry/exit is, so I expect that this would be a > > > > net loss in overall performance. > > > > > > We unconditionally have the preempt_count, its just not used much for > > > PREEMPT_COUNT=n kernels. > > > > We have the field, you mean? I might be missing something, but it still > > appears to me thta preempt_disable() does nothing for PREEMPT=n kernels. > > So what am I missing? > > There's another layer of accessors that can in fact manipulate the > preempt_count even for !PREEMPT_COUNT kernels. They are currently used > by things like pagefault_disable().
OK, fair enough. I am going to focus first on getting rid of (or at least greatly reducing) RCU's interrupt disabling on the user-kernel entry/exit paths, since that seems to be the biggest cost. 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/