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 ? Let's make sure the bug is not in RCU. After Al changes, RCU got slightly better under stress.
