On Wed, Nov 30, 2016 at 05:38:20PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 30, 2016 at 06:29:55AM -0800, Paul E. McKenney wrote:
> > We can, and you are correct that cond_resched() does not unconditionally
> > supply RCU quiescent states, and never has. Last time I tried to add
> > cond_resched_rcu_qs() semantics to cond_resched(), I got told "no",
> > but perhaps it is time to try again.
>
> Well, you got told: "ARRGH my benchmark goes all regress", or something
> along those lines. Didn't we recently dig out those commits for some
> reason or other?
Were "those commits" the benchmark or putting cond_resched_rcu_qs()
functionality into cond_resched()? Either way, no idea.
> Finding out what benchmark that was and running it against this patch
> would make sense.
Agreed, especially given that I believe cond_resched_rcu_qs() is lighter
weight than it used to be. No idea what benchmarks they were, though.
> Also, I seem to have missed, why are we going through this again?
People are running workloads that force long-running loops in the kernel,
which get them RCU CPU stall warning messages. My reaction has been
to insert cond_resched_rcu_qs() as needed, and Michal wondered why
cond_resched() couldn't just handle both scheduling latency and RCU
quiescent states. I remembered trying it, but not what the issue was.
So I posted the patch assuming that I would eventually either find out
what the issue was or that the issue no longer applied. ;-)
Thanx, Paul
