linux-kernel - Re: [PATCH net v2 0/2] Revert the 'socket

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Date:   Tue, 5 May 2020 08:20:50 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     SeongJae Park <sjpark@...zon.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jakub Kicinski <kuba@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        sj38.park@...il.com, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        SeongJae Park <sjpark@...zon.de>, snu@...zon.com,
        amit@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change



On 5/5/20 8:07 AM, SeongJae Park wrote:
> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@...gle.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.