On 07/15, Paul E. McKenney wrote: > > On Sun, Jul 12, 2015 at 01:36:01AM +0200, Oleg Nesterov wrote: > > Currently down_write/up_write calls synchronize_sched_expedited() > > twice which is evil. Change this code to rely on rcu-sync primitives. > > This avoids the _expedited "big hammer", and this can be faster in > > the contended case or even in the case when a single thread does > > down_write/up_write in a loop. > > But "evil" is such a strong word! ;-)
I tried to be convincive ;) But yes, to me this synchronize_sched_expedited() looks really annoying because it is not friendly to other workloads. > More seriously, introducing a read-side smp_mb() Sure, we can penalize the readers and simplify this code. But the main purpose of this primitive was "make the readers fast". I'll write another email about this... > My main concern would be the introduction of an exclusive lock for the > writer, but if you are hitting the write-side acquisition that hard, > perhaps you are having other bigger problems. Sorry, I don't really understand... could you explain? > Another concern would be performance in a workload where there are way > more readers than writers, but enough writers so that there is not much > more than one grace period's worth of spacing between them. In that case, > the readers are having to hit the global lock more frequently with this > change than in the original. Of course, this is true. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
