On 05/05/2015 02:35 PM, Paul E. McKenney wrote: > 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.
Interrupts are already disabled on kernel-user and kernel-guest switches. Paolo and I have patches to move a bunch of the calls to user_enter, user_exit, guest_enter, and guest_exit to places where interrupts are already disabled, so we do not need to disable them again. With those in place, the vtime calculations are the largest CPU user. I am working on those. -- All rights reversed -- 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/
