Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change

[Nuernberger, Stefan]

On Tue, 2020-05-05 at 13:54 +0200, SeongJae Park wrote:
> CC-ing stable@xxxxxxxxxxxxxxx and adding some more explanations.
> 
> On Tue, 5 May 2020 10:10:33 +0200 SeongJae Park <sjpark@xxxxxxxxxx>
> wrote:
> 
> > 
> > From: SeongJae Park <sjpark@xxxxxxxxx>
> > 
> > The commit 6d7855c54e1e ("sockfs: switch to ->free_inode()") made
> > the
> > deallocation of 'socket_alloc' to be done asynchronously using RCU,
> > as
> > same to 'sock.wq'.ÂÂAnd the following commit 333f7909a857
> > ("coallocate
> > socket_sq with socket itself") made those to have same life cycle.
> > 
> > The changes made the code much more simple, but also made
> > 'socket_alloc'
> > live longer than before.ÂÂFor the reason, user programs intensively
> > repeating allocations and deallocations of sockets could cause
> > memory
> > pressure on recent kernels.
> I found this problem on a production virtual machine utilizing 4GB
> memory while
> running lebench[1].ÂÂThe 'poll big' test of lebench opens 1000
> sockets, polls
> and closes those.ÂÂThis test is repeated 10,000 times.ÂÂTherefore it
> should
> consume only 1000 'socket_alloc' objects at once.ÂÂAs size of
> socket_alloc is
> about 800 Bytes, it's only 800 KiB.ÂÂHowever, on the recent kernels,
> it could
> consume up to 10,000,000 objects (about 8 GiB).ÂÂOn the test machine,
> I
> confirmed it consuming about 4GB of the system memory and results in
> OOM.
> 
> [1] https://github.com/LinuxPerfStudy/LEBench
> 
> > 
> > 
> > To avoid the problem, this commit reverts the changes.
> I also tried to make fixup rather than reverts, but I couldn't easily
> find
> simple fixup.ÂÂAs the commits 6d7855c54e1e and 333f7909a857 were for
> code
> refactoring rather than performance optimization, I thought
> introducing complex
> fixup for this problem would make no sense.ÂÂMeanwhile, the memory
> pressure
> regression could affect real machines.ÂÂTo this end, I decided to
> quickly
> revert the commits first and consider better refactoring later.
> 

While lebench might be exercising a rather pathological case, the
increase in memory pressure is real. I am concerned that the OOM killer
is actually engaging and killing off processes when there are lots of
resources already marked for release. This might be true for other
lazy/delayed resource deallocation, too. This has obviously just become
too lazy currently.

So for both reverts:

Reviewed-by: Stefan Nuernberger <snu@xxxxxxxxxx>

> 
> Thanks,
> SeongJae Park
> 
> > 
> > 
> > SeongJae Park (2):
> > Â Revert "coallocate socket_wq with socket itself"
> > Â Revert "sockfs: switch to ->free_inode()"
> > 
> > Âdrivers/net/tap.cÂÂÂÂÂÂ|ÂÂ5 +++--
> > Âdrivers/net/tun.cÂÂÂÂÂÂ|ÂÂ8 +++++---
> > Âinclude/linux/if_tap.h |ÂÂ1 +
> > Âinclude/linux/net.hÂÂÂÂ|ÂÂ4 ++--
> > Âinclude/net/sock.hÂÂÂÂÂ|ÂÂ4 ++--
> > Ânet/core/sock.cÂÂÂÂÂÂÂÂ|ÂÂ2 +-
> > Ânet/socket.cÂÂÂÂÂÂÂÂÂÂÂ| 23 ++++++++++++++++-------
> > Â7 files changed, 30 insertions(+), 17 deletions(-)
> > 



Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879