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. */
