On Sat, Sep 22, 2012 at 10:20:19PM +0000, Paul Walmsley wrote: > Hi Paul > > On Sat, 22 Sep 2012, Paul E. McKenney wrote: > > > Strangely enough, I believe that I have inadvertently fixed this in > > my -rcu tree: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/next > > > > Nevertheless, if you get a chance to try it, I would be interested to > > hear if my guess is correct. > > Yes, good news: the stall warnings go away with that branch. Very good! > > The trick is that a kthread drives the grace period in -rcu, regardless > > of whether or not there are callbacks. > > This is "rcu: Move quiescent-state forcing into kthread" ? Yep, plus the preceding commits moving grace-period initialization and cleanup into that same kthread. This was motivated by a bug report last February complaining about 200-microsecond latency spikes from RCU grace-period initialization. On systems with 4096 CPUs. Real-time response. It is far bigger than I thought. ;-) > Added some debugging into rcu_gp_kthread() after that commit and can > confirm that the quiescent-state forcing loop does start a few times when > there are zero callbacks pending (modulo any races in my measurement > code). Cool, thank you! Assuming it works, that indicates that there is long-term value to the fix for this problem. On larger systems, extra grace periods are not what you want, as their expense increases with the number of CPUs. > > However, the backport would not be something that -stable would be happy > > with, so I will be putting together a fix for mainline. This thing > > has been in the kernel since about 2004, not sure why you didn't hit > > it earlier. > > One other data point in that regard - noticed the warnings don't appear > when the board is booted with: > > commit 4fa3b6cb1bc8c14b81b4c8ffdfd3f2500a7e9367 > Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx> > Date: Tue Jun 5 15:53:53 2012 -0700 > > rcu: Fix qlen_lazy breakage You lost me on this one. This is already in mainline, so if you were using (say) 3.6-rc6, you would already have this commit applied. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
