On Tue, 5 May 2020 10:23:58 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > On Tue, May 05, 2020 at 09:25:06AM -0700, Eric Dumazet wrote: > > > > > > On 5/5/20 9:13 AM, SeongJae Park wrote: > > > On Tue, 5 May 2020 09:00:44 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> wrote: > > > > > >> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > >>> > > >>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > > >>> > > >>>> > > >>>> > > >>>> On 5/5/20 8:07 AM, SeongJae Park wrote: > > >>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> wrote: > > >>>>> > > >>>> > > >>>>>> Why do we have 10,000,000 objects around ? Could this be because of > > >>>>>> some RCU problem ? > > >>>>> > > >>>>> Mainly because of a long RCU grace period, as you guess. I have no idea how > > >>>>> the grace period became so long in this case. > > >>>>> > > >>>>> As my test machine was a virtual machine instance, I guess RCU readers > > >>>>> preemption[1] like problem might affected this. > > >>>>> > > >>>>> [1] https://www.usenix.org/system/files/conference/atc17/atc17-prasad.pdf > > If this is the root cause of the problem, then it will be necessary to > provide a hint to the hypervisor. Or, in the near term, avoid loading > the hypervisor the point that vCPU preemption is so lengthy. > > RCU could also provide some sort of pre-stall-warning notification that > some of the CPUs aren't passing through quiescent states, which might > allow the guest OS's userspace to take corrective action. > > But first, what are you doing to either confirm or invalidate the > hypothesis that this might be due to vCPU preemption? Nothing, I was just guessing. Sorry if this made you confused. > > > >>>>>> Once Al patches reverted, do you have 10,000,000 sock_alloc around ? > > >>>>> > > >>>>> Yes, both the old kernel that prior to Al's patches and the recent kernel > > >>>>> reverting the Al's patches didn't reproduce the problem. > > >>>>> > > >>>> > > >>>> I repeat my question : Do you have 10,000,000 (smaller) objects kept in slab caches ? > > >>>> > > >>>> TCP sockets use the (very complex, error prone) SLAB_TYPESAFE_BY_RCU, but not the struct socket_wq > > >>>> object that was allocated in sock_alloc_inode() before Al patches. > > >>>> > > >>>> These objects should be visible in kmalloc-64 kmem cache. > > >>> > > >>> Not exactly the 10,000,000, as it is only the possible highest number, but I > > >>> was able to observe clear exponential increase of the number of the objects > > >>> using slabtop. Before the start of the problematic workload, the number of > > >>> objects of 'kmalloc-64' was 5760, but I was able to observe the number increase > > >>> to 1,136,576. > > >>> > > >>> OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME > > >>> before: 5760 5088 88% 0.06K 90 64 360K kmalloc-64 > > >>> after: 1136576 1136576 100% 0.06K 17759 64 71036K kmalloc-64 > > >>> > > >> > > >> Great, thanks. > > >> > > >> How recent is the kernel you are running for your experiment ? > > > > > > It's based on 5.4.35. > > Is it possible to retest on v5.6? I have been adding various mechanisms > to make RCU keep up better with heavy callback overload. I will try soon! > > Also, could you please provide the .config? If either NO_HZ_FULL or > RCU_NOCB_CPU, please also provide the kernel boot parameters. NO_HZ_FULL is not set, but RCU_NOCB_CPU is y. I think I should check whether it's ok to share the full config and boot parameters. Please wait this. > > > >> Let's make sure the bug is not in RCU. > > > > > > One thing I can currently say is that the grace period passes at last. I > > > modified the benchmark to repeat not 10,000 times but only 5,000 times to run > > > the test without OOM but easily observable memory pressure. As soon as the > > > benchmark finishes, the memory were freed. > > > > > > If you need more tests, please let me know. > > > > I would ask Paul opinion on this issue, because we have many objects > > being freed after RCU grace periods. > > As always, "It depends." > > o If the problem is a too-long RCU reader, RCU is prohibited from > ending the grace period. The reader duration must be shortened, > and until it is shortened, there is nothing RCU can do. > > o In some special cases of the above, RCU can and does help, for > example, by enlisting the aid of cond_resched(). So perhaps > there is a long in-kernel loop that needs a cond_resched(). > > And perhaps RCU can help for some types of vCPU preemption. > > o As Al suggested offline and as has been discussed in the past, > it would not be hard to cause RCU to burn CPU to attain faster > grace periods during OOM events. This could be helpful, but only > given that RCU readers are completing in reasonable timeframes. Totally agreed. > > > If RCU subsystem can not keep-up, I guess other workloads will also suffer. > > If readers are not excessively long, RCU should be able to keep up. > (In the absence of misconfigurations, for example, both NO_HZ_FULL and > then binding all the rcuo kthreads to a single CPU on a 100-CPU system > or some such.) > > > Sure, we can revert patches there and there trying to work around the issue, > > but for objects allocated from process context, we should not have these problems. > > Agreed, let's get more info on what is happening to RCU. > > One approach is to shorten the RCU CPU stall warning timeout > (rcupdate.rcu_cpu_stall_timeout=10 for 10 seconds). I will also try this and let you know the results. Thanks, SeongJae Park > > Thanx, Paul
