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

[Manfred Spraul <manfred@colorfullife.com>]

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