On Tue, May 5, 2020 at 8:47 AM SeongJae Park sjpark@amazon.com wrote:
On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet eric.dumazet@gmail.com wrote:
On 5/5/20 8:07 AM, SeongJae Park wrote:
On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet edumazet@google.com 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 NAMEbefore: 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.