Re: [PATCH] rcu: eliminate rcu_data.last_qsctr

[Manfred Spraul <manfred@colorfullife.com>]

Oleg Nesterov wrote:

>last_qsctr is used in rcu_check_quiescent_state() exclusively.
>We can reset qsctr at the start of the grace period, and then
>just test qsctr against 0.
>
>  
>
It seems the patch got lost, I've updated it a bit and resent it to Andrew.

But: I think there is the potential for an even larger cleanup, although 
this would be more a rewrite:
Get rid of rcu_check_quiescent_state and instead use something like this 
in rcu_qsctr_inc:

static inline void rcu_qsctr_inc(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
        if (rdp->quiescbatch != rcp->cur) {
             /* a new grace period is running. And we are at a quiescent
              * point, so complete it
              */
             spin_lock(&rsp->lock);
             rdp->quiescbatch = rcp->cur;
             cpu_quiet(rdp->cpu, rcp, rsp);
            spin_unlock(&rsp->lock);
     }
}

It's just an idea, it needs testing on big systems - does reading from 
the global rcp from every schedule call cause any problems? The cache 
line is virtually read-only, so it shouldn't cause trashing, but who knows?

--
    Manfred