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

Randy Dunlap
2020-10-13T14:44:38Z

On 10/13/20 1:09 AM, Mike Rapoport wrote:
> On Mon, Oct 12, 2020 at 05:53:01PM +0800, Muchun Song wrote:
>> On Mon, Oct 12, 2020 at 5:24 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>>
>>> 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.
>>
>> I think that the consumer which consumes a lot of memory should be added
>> to the /proc/meminfo. This can help us know the user of large memory.
>>
>>>
>>> If you do not want 9.37 % of physical memory being possibly used by TCP,
>>> just change /proc/sys/net/ipv4/tcp_mem accordingly ?
>>
>> We are not complaining about TCP using too much memory, but how do
>> we know that TCP uses a lot of memory. When I firstly face this problem,
>> I do not know who uses the 25GB memory and it is not shown in the /proc/meminfo.
>> If we can know the amount memory of the socket buffer via /proc/meminfo, we
>> may not need to spend a lot of time troubleshooting this problem. Not everyone
>> knows that a lot of memory may be used here. But I believe many people
>> should know /proc/meminfo to confirm memory users.
>
> If I undestand correctly, the problem you are trying to solve is to
> simplify troubleshooting of memory usage for people who may not be aware
> that networking stack can be a large memory consumer.
>
> For that a paragraph in 'man 5 proc' maybe a good start:
>
>>From ddbcf38576d1a2b0e36fe25a27350d566759b664 Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <rppt@linux.ibm.com>
> Date: Tue, 13 Oct 2020 11:07:35 +0300
> Subject: [PATCH] proc.5: meminfo: add not anout network stack memory
> consumption
>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> man5/proc.5 | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/man5/proc.5 b/man5/proc.5
> index ed309380b..8414676f1 100644
> --- a/man5/proc.5
> +++ b/man5/proc.5
> @@ -3478,6 +3478,14 @@ Except as noted below,
> all of the fields have been present since at least Linux 2.6.0.
> Some fields are displayed only if the kernel was configured
> with various options; those dependencies are noted in the list.
> +.IP
> +Note that significant part of memory allocated by the network stack
> +is not accounted in the file.
> +The memory consumption of the network stack can be queried
> +using
> +.IR /proc/net/sockstat
> +or
> +.BR ss (8)
> .RS
> .TP
> .IR MemTotal " %lu"

Hi Mike,

Could you tell us what units those values are in?
or is that already explained somewhere else?

thanks.
--
~Randy