Re: [RFC PATCH v2 3/5] futex: Throughput-optimized (TO) futexes

From: Waiman Long
Date: Thu Sep 22 2016 - 13:21:30 EST

On 09/22/2016 09:23 AM, Peter Zijlstra wrote:
On Tue, Sep 20, 2016 at 09:42:41AM -0400, Waiman Long wrote:
+ * Spinning threshold before enabling lock handoff.
+ * Each sleep will decrement the threshold by 1/32 of the start value.
+ */
+#define TO_SPIN_THRESHOLD (1<< 13)
Argh, we should really get rid of those stupid numbers. Wasn't there a
patch set working on implementing paravirt functions that would make all
this fixable in a sane way?

These thresholds are not paravirt related. They are there to determine when we should activate explicit lock handoff when the waiter is waiting for too long. Yes, it is arbitrary. The numbers are on the high side as setting them too low will limit lock stealing and hence overall performance. The FUTEX_WAITERS bit is only turned on when either the top waiter is sleeping or a lock handoff is needed. Otherwise, it is off to encourage lock stealing in the userspace as well as in the kernel.