On Fri, Aug 12, 2011 at 04:45:59PM -0700, Michel Lespinasse wrote:
> On Fri, Aug 12, 2011 at 10:27 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > Or maybe I make rcu_cookie_gp_elapsed() take only one cookie and
> > compare it to the current cookie. This would save a bit of code in
> > the TINY cases:
> >
> > rcu_get_gp_cookie(&pagep->rcucookie);
> > . . .
> >
> > if (!rcu_cookie_gp_elapsed(&pagep->rcucookie))
> > synchronize_rcu();
>
> Agree this looks nicer that having the second cookie on the stack. As
> you said, this does not allow us to compare two past points in time,
> but I really don't see a use case for that.
And actually hand-writing the code got me the following API:
struct rcu_cookie;
void rcu_get_gp_cookie(struct rcu_cookie *rcp);
void rcu_gp_cookie_elapsed(struct rcu_cookie *rcp);
For TREE{_PREEMPT,}_RCU these are both external calls (#include hell
and all that). For TINY{_PREEMPT,}_RCU they are both trivial inlineable
functions.
> > How long would there normally be between recording the cookie and
> > checking for the need for a grace period? One disk access? One HZ?
> > Something else?
>
> I would expect >>10 seconds in the normal case ? I'm not sure how much
> lower this may get in adverse workloads. Andrea ?
>>10 seconds would be way more than enough to allow this to work well.
But if we are getting much below 100 milliseconds, we need to rethink
this.
Thanx, Paul
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