On Wed, Dec 13, 2017 at 10:17:41AM +0800, Huang, Ying wrote: > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> writes: > > > On Tue, Dec 12, 2017 at 09:12:20AM +0800, Huang, Ying wrote: > >> Hi, Pual, > >> > >> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> writes: > >> > >> > On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote: > >> >> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes: > >> >> > >> >> > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@xxxxxxxxx> wrote: > >> >> > > >> >> >> > Why do we need srcu here? Is it enough with rcu like below? > >> >> >> > > >> >> >> > It might have a bug/room to be optimized about performance/naming. > >> >> >> > I just wanted to show my intention. > >> >> >> > >> >> >> Yes. rcu should work too. But if we use rcu, it may need to be called > >> >> >> several times to make sure the swap device under us doesn't go away, for > >> >> >> example, when checking si->max in __swp_swapcount() and > >> >> >> add_swap_count_continuation(). And I found we need rcu to protect swap > >> >> >> cache radix tree array too. So I think it may be better to use one > >> >> >> calling to srcu_read_lock/unlock() instead of multiple callings to > >> >> >> rcu_read_lock/unlock(). > >> >> > > >> >> > Or use stop_machine() ;) It's very crude but it sure is simple. Does > >> >> > anyone have a swapoff-intensive workload? > >> >> > >> >> Sorry, I don't know how to solve the problem with stop_machine(). > >> >> > >> >> The problem we try to resolved is that, we have a swap entry, but that > >> >> swap entry can become invalid because of swappoff between we check it > >> >> and we use it. So we need to prevent swapoff to be run between checking > >> >> and using. > >> >> > >> >> I don't know how to use stop_machine() in swapoff to wait for all users > >> >> of swap entry to finish. Anyone can help me on this? > >> > > >> > You can think of stop_machine() as being sort of like a reader-writer > >> > lock. The readers can be any section of code with preemption disabled, > >> > and the writer is the function passed to stop_machine(). > >> > > >> > Users running real-time applications on Linux don't tend to like > >> > stop_machine() much, but perhaps it is nevertheless the right tool > >> > for this particular job. > >> > >> Thanks a lot for explanation! Now I understand this. > >> > >> Another question, for this specific problem, I think both stop_machine() > >> based solution and rcu_read_lock/unlock() + synchronize_rcu() based > >> solution work. If so, what is the difference between them? I guess rcu > >> based solution will be a little better for real-time applications? So > >> what is the advantage of stop_machine() based solution? > > > > The stop_machine() solution places similar restrictions on readers as > > does rcu_read_lock/unlock() + synchronize_rcu(), if that is what you > > are asking. > > > > More precisely, the stop_machine() solution places exactly the > > same restrictions on readers as does preempt_disable/enable() and > > synchronize_sched(). > > > > I would expect stop_machine() to be faster than either synchronize_rcu() > > synchronize_sched(), or synchronize_srcu(), but stop_machine() operates > > by making each CPU spin with interrupts until all the other CPUs arrive. > > This normally does not make real-time people happy. > > > > An compromise position is available in the form of > > synchronize_rcu_expedited() and synchronize_sched_expedited(). These > > are faster than their non-expedited counterparts, and only momentarily > > disturb each CPU, rather than spinning with interrupts disabled. However, > > stop_machine() is probably a bit faster. > > > > Finally, syncrhonize_srcu_expedited() is reasonably fast, but > > avoids disturbing other CPUs. Last I checked, not quite as fast as > > synchronize_rcu_expedited() and synchronize_sched_expedited(), though. > > > > You asked! ;-) > > Thanks a lot Paul! That exceeds my expectation! > > The performance of swapoff() isn't very important, probably it's not > necessary to accelerate it at the cost of realtime. I think it is > better to use a rcu or srcu based solution. I think the cost at reader > side should be almost same between rcu and srcu? To use srcu, we need > to select CONFIG_SRCU when CONFIG_SWAP is enabled in Kconfig. I think > that should be OK? The thing to do is to try SRCU and see if you can see significant performance degradation. Given that there is swapping involved, I would be surprised if the added read-side overhead of SRCU was even measurable, but then again I have been surprised before. And yes, just select CONFIG_SRCU when you need it. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
