Re: [External] Re: [PATCH] mm: proc: add Sock to /proc/meminfo

Eric Dumazet
2020-10-12T09:24:13Z

On 10/12/20 10:39 AM, Muchun Song wrote:
> On Mon, Oct 12, 2020 at 3:42 PM Eric Dumazet <edumazet@google.com> wrote:
>>
>> On Mon, Oct 12, 2020 at 6:22 AM Muchun Song <songmuchun@bytedance.com> wrote:
>>>
>>> On Mon, Oct 12, 2020 at 2:39 AM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>>>
>>>> On Sat, Oct 10, 2020 at 3:39 AM Muchun Song <songmuchun@bytedance.com> wrote:
>>>>>
>>>>> The amount of memory allocated to sockets buffer can become significant.
>>>>> However, we do not display the amount of memory consumed by sockets
>>>>> buffer. In this case, knowing where the memory is consumed by the kernel
>>>>
>>>> We do it via `ss -m`. Is it not sufficient? And if not, why not adding it there
>>>> rather than /proc/meminfo?
>>>
>>> If the system has little free memory, we can know where the memory is via
>>> /proc/meminfo. If a lot of memory is consumed by socket buffer, we cannot
>>> know it when the Sock is not shown in the /proc/meminfo. If the unaware user
>>> can't think of the socket buffer, naturally they will not `ss -m`. The
>>> end result
>>> is that we still don’t know where the memory is consumed. And we add the
>>> Sock to the /proc/meminfo just like the memcg does('sock' item in the cgroup
>>> v2 memory.stat). So I think that adding to /proc/meminfo is sufficient.
>>>
>>>>
>>>>> static inline void __skb_frag_unref(skb_frag_t *frag)
>>>>> {
>>>>> - put_page(skb_frag_page(frag));
>>>>> + struct page *page = skb_frag_page(frag);
>>>>> +
>>>>> + if (put_page_testzero(page)) {
>>>>> + dec_sock_node_page_state(page);
>>>>> + __put_page(page);
>>>>> + }
>>>>> }
>>>>
>>>> You mix socket page frag with skb frag at least, not sure this is exactly
>>>> what you want, because clearly skb page frags are frequently used
>>>> by network drivers rather than sockets.
>>>>
>>>> Also, which one matches this dec_sock_node_page_state()? Clearly
>>>> not skb_fill_page_desc() or __skb_frag_ref().
>>>
>>> Yeah, we call inc_sock_node_page_state() in the skb_page_frag_refill().
>>> So if someone gets the page returned by skb_page_frag_refill(), it must
>>> put the page via __skb_frag_unref()/skb_frag_unref(). We use PG_private
>>> to indicate that we need to dec the node page state when the refcount of
>>> page reaches zero.
>>>
>>
>> Pages can be transferred from pipe to socket, socket to pipe (splice()
>> and zerocopy friends...)
>>
>> If you want to track TCP memory allocations, you always can look at
>> /proc/net/sockstat,
>> without adding yet another expensive memory accounting.
>
> The 'mem' item in the /proc/net/sockstat does not represent real
> memory usage. This is just the total amount of charged memory.
>
> For example, if a task sends a 10-byte message, it only charges one
> page to memcg. But the system may allocate 8 pages. Therefore, it
> does not truly reflect the memory allocated by the above memory
> allocation path. We can see the difference via the following message.
>
> cat /proc/net/sockstat
> sockets: used 698
> TCP: inuse 70 orphan 0 tw 617 alloc 134 mem 13
> UDP: inuse 90 mem 4
> UDPLITE: inuse 0
> RAW: inuse 1
> FRAG: inuse 0 memory 0
>
> cat /proc/meminfo | grep Sock
> Sock: 13664 kB
>
> The /proc/net/sockstat only shows us that there are 17*4 kB TCP
> memory allocations. But apply this patch, we can see that we truly
> allocate 13664 kB(May be greater than this value because of per-cpu
> stat cache). Of course the load of the example here is not high. In
> some high load cases, I believe the difference here will be even
> greater.
>

This is great, but you have not addressed my feedback.

TCP memory allocations are bounded by /proc/sys/net/ipv4/tcp_mem

Fact that the memory is forward allocated or not is a detail.

If you think we must pre-allocate memory, instead of forward allocations,
your patch does not address this. Adding one line per consumer in /proc/meminfo looks
wrong to me.

If you do not want 9.37 % of physical memory being possibly used by TCP,
just change /proc/sys/net/ipv4/tcp_mem accordingly ?