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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 5 May 2020 09:00:44 -0700
Message-ID: <CANn89iLHV2wyhk6-d6j_4=Ns01AEE5HSA4Qu3LO0gqKgcG81vQ@mail.gmail.com>
Subject: Re: Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change
To:     SeongJae Park <sjpark@amazon.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        David Miller <davem@davemloft.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jakub Kicinski <kuba@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        sj38.park@gmail.com, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        SeongJae Park <sjpark@amazon.de>, snu@amazon.com,
        amit@kernel.org, stable@vger.kernel.org
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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 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.