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

[Muchun Song]

On Mon, Oct 12, 2020 at 5:24 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>
>
>
> On 10/12/20 10:39 AM, Muchun Song wrote:
> > On Mon, Oct 12, 2020 at 3:42 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >>
> >> On Mon, Oct 12, 2020 at 6:22 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >>>
> >>> On Mon, Oct 12, 2020 at 2:39 AM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> >>>>
> >>>> On Sat, Oct 10, 2020 at 3:39 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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.

Thanks.

>
>


-- 
Yours,
Muchun