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! ;-) 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>
