Re: [bisected] pre-3.16 regression on open() scalability

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 17, 2014 at 07:27:31PM -0700, Andi Kleen wrote:
> > OK.  What would you suggest instead?  If all we do is to revert the
> 
> Hang checker should have two timer phases:
> 
> Timer fires first time:
> - Save context switch counter on that. Force a reschedule to some
> work queue. Rearm timer
> 
> Timer fires again:
> - Check reschedule count. If the reschedule count changed
> it was a real hang, otherwise ignore.

I could take that approach, but the RT guys aren't going to thank me for
the wakeup associated with the work queue.  I suppose that I could use
one approach for real-time workloads and your workqueue-base approach
for other workloads.

Still, I bet I can drop the common-case cond_resched() overhead to a
single read of a per-CPU variable and a branch.  But yes, that would be
in response to the second phase, FWIW.  If that measurably too much
overhead, then one thing for realtime and another otherwise.

							Thanx, Paul