Re: [PATCH 4/5] rcu: join rcu_ctrlblk and rcu_state

[Paul E. McKenney]

On Tue, Jan 10, 2006 at 09:44:56PM +0100, Manfred Spraul wrote:
> [I haven't read the diff, just a short comment]
> 
> Dipankar Sarma wrote:
> 
> >rcu_state came over from Manfred's RCU_HUGE patch IIRC. I don't
> >think it is necessary to allocate rcu_state separately in the
> >current mainline RCU code. So, the patch looks OK to me, but
> >Manfred might see something that I am not seeing.
> >
> > 
> >
> The two-level rcu code was never merged, I still plan to clean it up.
> 
> But the idea of splitting the control block and the state is used in the 
> current code:
> - __rcu_pending() is the hot path, it only performs a read access to 
> rcu_ctrlblk.
> - write accesses to the rcu_ctrlblk are really rare, they only happen 
> when a new batch is started. Especially: independant from the number of 
> cpus.
> 
> Write access to the rcu_state are common:
> - each cpu must write once in each cycle to update it's cpu mask.
> - The last cpu then completes the quiescent cycle.
> 
> The idea is that rcu_state is more or less write-only and rcu_state is 
> read-only. Theoretically, rcu_state could be shared in all cpus caches, 
> and there will be only one invalidate when a new batch is started. Thus 
> no cacheline trashing due to rcu_pending calls.
> I think it would be safer to keep the two state counters in a separate 
> cacheline from the spinlock and the cpu mask, but I don't have any hard 
> numbers. IIRC the problems with the large SGI systems disappered, and 
> everyone was happy. No real benchmark comparisons were made.

Good point!

But doesn't the ____cacheline_maxaligned_in_smp directive on the "lock"
field take care of this?  Here is the structure:

/* Global control variables for rcupdate callback mechanism. */
struct rcu_ctrlblk {
        long    cur;            /* Current batch number.                      */
        long    completed;      /* Number of the last completed batch         */
        int     next_pending;   /* Is the next batch already waiting?         */

	spinlock_t      lock    ____cacheline_internodealigned_in_smp;
	cpumask_t       cpumask; /* CPUs that need to switch in order    */
				 /* for current batch to proceed.        */
} ____cacheline_internodealigned_in_smp;

If this does not cover this case, what more is needed?

							Thanx, Paul
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