Re: [PATCH tip/core/rcu 06/27] rcu: Mark task as .need_qs less aggressively

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

On Tue, Jun 26, 2018 at 07:08:12PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 25, 2018 at 05:34:52PM -0700, Paul E. McKenney wrote:
> > If any scheduling-clock interrupt interrupts an RCU-preempt read-side
> > critical section, the interrupted task's ->rcu_read_unlock_special.b.need_qs
> > field is set.  This causes the outermost rcu_read_unlock() to incur the
> > extra overhead of calling into rcu_read_unlock_special().  This commit
> > reduces that overhead by setting ->rcu_read_unlock_special.b.need_qs only
> > if the grace period has been in effect for more than one second.
> 
> Even less agressive is never setting it at all.

True, but if the CPU has been in an RCU read-side critical section for
a full second (which is the case with high probability when .b.need_qs
is set after this change), we might want to respond to the end of that
critical section sooner rather than later.

> Changelog fails to explain why not setting it every tick is correct, nor
> why 1s is a 'safe' value to use.

The RCU CPU stall warning cannot be set to less than 3s, so 1s is
reasonable.  It is a tradeoff -- setting it lower causes a greater
fraction of RCU read-side critical sections to incur extra overhead at
rcu_read_unlock() time, while setting it higher keeps a lazy approach
to reporting the quiescent state to core RCU for longer critical sections.

The upcoming RCU-bh/RCU-preempt/RCU-sched consolidation will raise
contention and overhead, so this is one of several things done to
lower overhead and contention to compensate for that.

							Thanx, Paul