Messages in this thread | | | Date | Sun, 17 May 2009 15:08:35 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods |
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On Mon, May 18, 2009 at 12:02:23AM +0400, Evgeniy Polyakov wrote: > Hi.
Hello, Evgeniy!
> On Sun, May 17, 2009 at 12:11:41PM -0700, Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > Fifth cut of "big hammer" expedited RCU grace periods. This uses per-CPU > > kthreads that are scheduled in parallel by a call to smp_call_function() > > by yet another kthread. The synchronize_sched(), synchronize_rcu(), > > and synchronize_bh() primitives wake this kthread up and then wait for > > it to force the grace period. > > I'm curious, but doesn't the fact that registered 'barrier' callback is > invoked mean grace period completion? I.e. why to bother with > rescheduling, waiting for thread to complete and so on, when we only > care in the fact that 'barrier' callback is invoked, and thus all > previous ones are completed? > Or it is done just for the simplicity, since all rescheduling machinery > already manages the rcu bits correctly, so you do not want to put it > directly into 'barrier' callback?
It is a short-term expedient course of action. Longer term, I will drop rcuclassic in favor of rcutree, and then merge rcupreempt into rcutree. I will then add machinery to rcutree to handle expedited grace periods (somewhat) more naturally. Trying to expedite three very different RCU implementations seems a bit silly, hence the current off-on-the-side approach.
But even then I will avoid relying on a "barrier" callback, or, indeed, any sort of callback, because we don't want expedited grace periods to have to wait on invocation of earlier RCU callbacks. There will thus not be a call_rcu_expedited(), at least not unless someone comes up with a -really- compelling reason why.
But the exercise of going through several possible implementations was quite useful, as I learned a number of things that will improve the eventual rcutree implementation. Like the fact that expedited grace periods don't want to be waiting on invocation of prior callbacks. ;-)
And rcutiny is, as always, as special case. Here is the implementation of synchronize_rcu_expedited() in rcutiny:
void synchronize_rcu_expedited(void) { }
Or even:
#define synchronize_rcu_expedited synchronize_rcu
;-)
Thanx, Paul
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