Re: [PATCH v3] memcg: add charging of already allocated slab objects

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Aug 30, 2024 at 11:07:32AM GMT, Vlastimil Babka wrote:
> On 8/29/24 19:53, Shakeel Butt wrote:
> > At the moment, the slab objects are charged to the memcg at the
> > allocation time. However there are cases where slab objects are
> > allocated at the time where the right target memcg to charge it to is
> > not known. One such case is the network sockets for the incoming
> > connection which are allocated in the softirq context.
> > 
> > Couple hundred thousand connections are very normal on large loaded
> > server and almost all of those sockets underlying those connections get
> > allocated in the softirq context and thus not charged to any memcg.
> > However later at the accept() time we know the right target memcg to
> > charge. Let's add new API to charge already allocated objects, so we can
> > have better accounting of the memory usage.
> > 
> > To measure the performance impact of this change, tcp_crr is used from
> > the neper [1] performance suite. Basically it is a network ping pong
> > test with new connection for each ping pong.
> > 
> > The server and the client are run inside 3 level of cgroup hierarchy
> > using the following commands:
> > 
> > Server:
> >  $ tcp_crr -6
> > 
> > Client:
> >  $ tcp_crr -6 -c -H ${server_ip}
> > 
> > If the client and server run on different machines with 50 GBPS NIC,
> > there is no visible impact of the change.
> > 
> > For the same machine experiment with v6.11-rc5 as base.
> > 
> >           base (throughput)     with-patch
> > tcp_crr   14545 (+- 80)         14463 (+- 56)
> > 
> > It seems like the performance impact is within the noise.
> > 
> > Link: https://github.com/google/neper [1]
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Thanks, pushed to slab/for-next for test coverage, hopefully net people will
> ack.
> 
> Also one thing:
> 
> We should add some kernel doc for this, no? Explaining when people are
> supposed to use this, that objects from KMALLOC_NORMAL will be ignored, and
> what the return value means (including where it's faked to be true).
> 

Yes this makes sense. I will add this info similar to the kmalloc()
have. Should I send a v4 with this details?