On Fri, 29 May 2015 15:11:59 -0400 Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > Hi Clark, > > On Fri, May 29, 2015 at 10:48:15AM -0500, Clark Williams wrote: > > @@ -5845,7 +5845,7 @@ void mem_cgroup_swapout(struct page *page, > > swp_entry_t entry) page_counter_uncharge(&memcg->memory, 1); > > > > /* XXX: caller holds IRQ-safe mapping->tree_lock */ > > - VM_BUG_ON(!irqs_disabled()); > > + VM_BUG_ON(!spin_is_locked(&page_mapping(page)->tree_lock)); > > > > mem_cgroup_charge_statistics(memcg, page, -1); > > It's not about the lock, it's about preemption. The charge statistics OK, I just lost my bet with Clark. He said it was about preemption, and I said it was about the lock ;-) > use __this_cpu operations and they're updated from process context and > interrupt context both. > > This function really should do a local_irq_save(). I only added the > VM_BUG_ON() to document that we know the caller is holding an IRQ-safe > lock and so we don't need to bother with another level of IRQ saving. > > So how does this translate to RT? I don't know. But if switching to > explicit IRQ toggling would help you guys out we can do that. It is > in the swapout path after all, the optimization isn't that important. You only need to prevent this from preempting with other users here, right? RT provides a "local_lock_irqsave(var)" which on vanilla linux will do a local_irq_save(), but more importantly, it provides documentation of what that local_irq_save is about (the var). On -rt, that turns into a migrate disable, plus grabbing of the rt_mutex(var). Thus, the process wont migrate from that CPU, but may be preempted. If another process (or interrupt thread, as in -rt interrupts run as preemptable threads) tries to do a local_lock(var) on the same var, it will block. Basically, you get the same serialization in both, but you don't cause latencies in -rt. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
