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    SubjectRe: Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change
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    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 NAME
    before: 5760 5088 88% 0.06K 90 64 360K kmalloc-64
    after: 1136576 1136576 100% 0.06K 17759 64 71036K kmalloc-64


    Thanks,
    SeongJae Park

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