On Tue, May 05, 2020 at 09:37:42AM -0700, Eric Dumazet wrote: > > > On 5/5/20 9:31 AM, Eric Dumazet wrote: > > > > > > On 5/5/20 9:25 AM, 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 > >>>>>>> > >>>>>>>> > >>>>>>>> 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. > >>> > >>>> > >>>> 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. > >> > >> If RCU subsystem can not keep-up, I guess other workloads will also suffer. > >> > >> 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. > >> > > > > I wonder if simply adjusting rcu_divisor to 6 or 5 would help > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index d9a49cd6065a20936edbda1b334136ab597cde52..fde833bac0f9f81e8536211b4dad6e7575c1219a 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -427,7 +427,7 @@ module_param(qovld, long, 0444); > > static ulong jiffies_till_first_fqs = ULONG_MAX; > > static ulong jiffies_till_next_fqs = ULONG_MAX; > > static bool rcu_kick_kthreads; > > -static int rcu_divisor = 7; > > +static int rcu_divisor = 6; > > module_param(rcu_divisor, int, 0644); > > > > /* Force an exit from rcu_do_batch() after 3 milliseconds. */ > > > > To be clear, you can adjust the value without building a new kernel. > > echo 6 >/sys/module/rcutree/parameters/rcu_divisor Worth a try! If that helps significantly, I have some ideas for updating that heuristic, such as checking for sudden increases in the number of pending callbacks. But I would really also like to know whether there are long readers and whether v5.6 fares better. Thanx, Paul
