Re: [PATCH net-next 0/2] sctp: fully support memory accounting

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Mon, Apr 01, 2019 at 07:31:10AM -0400, Neil Horman wrote:
> On Sun, Mar 31, 2019 at 04:53:45PM +0800, Xin Long wrote:
> > sctp memory accounting is added in this patchset by using
> > these kernel APIs on send side:
> > 
> >   - sk_mem_charge()
> >   - sk_mem_uncharge()
> >   - sk_wmem_schedule()
> >   - sk_under_memory_pressure()
> >   - sk_mem_reclaim()
> > 
> > and these on receive side:
> > 
> >   - sk_mem_charge()
> >   - sk_mem_uncharge()
> >   - sk_rmem_schedule()
> >   - sk_under_memory_pressure()
> >   - sk_mem_reclaim()
> > 
> > With sctp memory accounting, we can limit the memory allocation by
> > either sysctl:
> > 
> >   # sysctl -w net.sctp.sctp_mem="10 20 50"
> > 
> > or cgroup:
> > 
> >   # echo $((8<<14)) > \
> >     /sys/fs/cgroup/memory/sctp_mem/memory.kmem.tcp.limit_in_bytes
> > 
> > When the socket is under memory pressure, the send side will block
> > and wait, while the receive side will renege or drop.
> > 
> > Xin Long (2):
> >   sctp: implement memory accounting on tx path
> >   sctp: implement memory accounting on rx path
> > 
> >  include/net/sctp/sctp.h |  2 +-
> >  net/sctp/sm_statefuns.c |  6 ++++--
> >  net/sctp/socket.c       | 10 ++++++++--
> >  net/sctp/ulpevent.c     | 19 ++++++++-----------
> >  net/sctp/ulpqueue.c     |  3 ++-
> >  5 files changed, 23 insertions(+), 17 deletions(-)
> > 
> > -- 
> > 2.1.0
> > 
> > 
> I don't have a problem with either of these patches in terms of altering memory
> accounting, but SCTP has the notion of accounting buffers based on either
> sockets space or association space (based on the sndbuf_policy and rcvbuf_policy
> sysctls).  This patch eliminates them.  I don't see this patch addressing either
> the removal of that functionality (as the proposed accounting scheme renders
> those sysctls useless and ignored, which may cause regressions in some
> environments), nor does it address the possibiliy of one association starving
> others on the same socket when they share the same socket level accounting.  I
> think you need to look how to address that (either by re-adding the ability to
> account in either case based on the sysctls, or deprecating eliminating the
> sysctls and addressing the starvation issue.

That's not how I'm reading these. All original conditions are still
there while these patches are adding a couple of restrictions more.
What that means is that they are adding a ceiling to it, even if the
limits are per socket or per assoc. Considering the idea of the cgroup
limit being added here, it makes sense to me. If the cgroup is
configured to allow at most X MB, it doesn't matter how that is
allocated. That's a sysadmin task then, to adjust the other sysctls
(net.sctp.sctp_mem & cia) and balance the usage, be it per socket or
per asoc.

Cheers,
  Marcelo