Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation

[Manfred Spraul]

Paul E. McKenney wrote:
> > > + */
> > > +struct rcu_node {
> > > +	spinlock_t lock;
> > > +	unsigned long	qsmask;	/* CPUs or groups that need to switch in      */
> > > +				/*  order for current grace period to proceed.*/
> > > +	unsigned long	qsmaskinit;
> > > +				/* Per-GP initialization for qsmask.	      */
> > >   
> > >       
> > I'm not sure if a bitmap is the right storage. If I understand the code 
> > correctly, it contains two information:
> > 1) If the bitmap is clear, then all cpus have completed whatever they need 
> > to do.
> > A counter is more efficient than a bitmap. Especially: It would allow to 
> > choose the optimal fan-out, independent from 32/64 bits.
> > 2) The information if the current cpu must do something to complete the 
> > current period.non
> > This is a local information, usually (always?) only the current cpu needs 
> > to know if it must do something.
> > But this doesn't need to be stored in a shared structure, the information 
> > could be stored in a per-cpu structure.
> >     
>
> I am using the bitmap in force_quiescent_state() to work out who to
> check dynticks and who to send reschedule IPIs to.  I could scan all
> of the per-CPU rcu_data structures, but am assuming that after a few
> jiffies there would typically be relatively few CPUs still needing to do
> a quiescent state.  Given this assumption, on systems with large numbers
> of CPUs, scanning the bitmask greatly reduces the number of cache misses
> compared to scanning the rcu_data structures.
>
>   
It's an optimization question: What is rarer? force_quiescent_state() or 
"normal" cpu_quiet calls.
You have optimized for force_quiescent_state(), I have optimized for 
"normal" cpu_quiet calls. [ok, I admit: force_quiescent_state() is still 
missing in my code].
Do you have any statistics?

--
   Manfred
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