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

Muchun Song
2020-10-12T08:40:07Z

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.

--
Yours,
Muchun