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

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Oct 13, 2020 at 4:09 PM Mike Rapoport <rppt@kernel.org> 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.

Yeah, you are right. Although the information provided by /proc/net/sockstat
is not accurate, it can also provide some valuable information. I think that it
might be better if we can add a total amount socket buffer to /proc/meminfo.
The amount socket buffer statistics can be from /proc/net/sockstat directly.

Thanks.

>
> 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"
> --
> 2.25.4
>
>


-- 
Yours,
Muchun