Re: [PATCH] a local-timer-free version of RCU

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Nov 08, 2010 at 11:38:32AM -0800, Paul E. McKenney wrote:
> On Mon, Nov 08, 2010 at 04:32:17PM +0100, Frederic Weisbecker wrote:
> > So, this looks very scary for performances to add rcu_read_lock() in
> > preempt_disable() and local_irq_save(), that notwithstanding it won't
> > handle the "raw" rcu sched implicit path.
> 
> Ah -- I would arrange for the rcu_read_lock() to be added only in the
> dyntick-hpc case.  So no effect on normal builds, overhead is added only
> in the dyntick-hpc case.



Yeah sure, but I wonder if the resulting rcu config will have a
large performance impact because of that.

In fact, my worry is: if the last resort to have a sane non-timer based
rcu is to bloat fast path functions like preempt_disable() or local_irq...
(that notwithstanding we have a bloated rcu_read_unlock() on this rcu config
because of its main nature), wouldn't it be better to eventually pick the
syscall/exception tweaked fast path version?

Perhaps I'll need to measure the impact of both, but I suspect I'll get
controversial results depending on the workload.


 
> > There is also my idea from the other discussion: change rcu_read_lock_sched()
> > semantics and map it to rcu_read_lock() in this rcu config (would be a nop
> > in other configs). So every users of rcu_dereference_sched() would now need
> > to protect their critical section with this.
> > Would it be too late to change this semantic?
> 
> I was expecting that we would fold RCU, RCU bh, and RCU sched into
> the same set of primitives (as Jim Houston did), but again only in the
> dyntick-hpc case.


Yeah, the resulting change must be NULL in others rcu configs.



> However, rcu_read_lock_bh() would still disable BH,
> and similarly, rcu_read_lock_sched() would still disable preemption.



Probably yeah, otherwise there will be a kind of sense split against
the usual rcu_read_lock() and everybody will be confused.

Perhaps we need a different API for the underlying rcu_read_lock()
call in the other flavours when preempt is already disabled or
bh is already disabled:

	rcu_enter_read_lock_sched();
	__rcu_read_lock_sched();
	rcu_start_read_lock_sched();

	(same for bh)

Hmm...


> > What is scary with this is that it also changes rcu sched semantics, and users
> > of call_rcu_sched() and synchronize_sched(), who rely on that to do more
> > tricky things than just waiting for rcu_derefence_sched() pointer grace periods,
> > like really wanting for preempt_disable and local_irq_save/disable, those
> > users will be screwed... :-(  ...unless we also add relevant rcu_read_lock_sched()
> > for them...
> 
> So rcu_read_lock() would be the underlying primitive.  The implementation
> of rcu_read_lock_sched() would disable preemption and then invoke
> rcu_read_lock().  The implementation of rcu_read_lock_bh() would
> disable BH and then invoke rcu_read_lock().  This would allow
> synchronize_rcu_sched() and synchronize_rcu_bh() to simply invoke
> synchronize_rcu().
> 
> Seem reasonable?


Perfect. That could be further optimized with what I said above but
other than that, that's what I was thinking about.