Message-ID: <CAAVpQUB8DzEjfc42dMRmGA0mgsHrCtXkPnQXZpG+EQq28xOXCg@mail.gmail.com>
Date: Thu, 4 Sep 2025 13:29:08 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Neal Cardwell <ncardwell@...gle.com>, Willem de Bruijn <willemb@...gle.com>, 
	Mina Almasry <almasrymina@...gle.com>, Kuniyuki Iwashima <kuni1840@...il.com>, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v4 bpf-next/net 5/5] selftest: bpf: Add test for SK_BPF_MEMCG_SOCK_ISOLATED.

On Thu, Sep 4, 2025 at 12:48 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 9/4/25 9:45 AM, Kuniyuki Iwashima wrote:
> > On Wed, Sep 3, 2025 at 10:51 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> >>
> >> On 9/3/25 10:08 AM, Kuniyuki Iwashima wrote:
> >>> On Wed, Sep 3, 2025 at 9:59 AM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
> >>>>
> >>>> On Tue, Sep 2, 2025 at 1:49 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
> >>>>>
> >>>>> On Tue, Sep 2, 2025 at 1:26 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> >>>>>>
> >>>>>> On 8/28/25 6:00 PM, Kuniyuki Iwashima wrote:
> >>>>>>> The test does the following for IPv4/IPv6 x TCP/UDP sockets
> >>>>>>> with/without BPF prog.
> >>>>>>>
> >>>>>>>      1. Create socket pairs
> >>>>>>>      2. Send a bunch of data that requires more than 256 pages
> >>>>>>>      3. Read memory_allocated from the 3rd column in /proc/net/protocols
> >>>>>>>      4. Check if unread data is charged to memory_allocated
> >>>>>>>
> >>>>>>> If BPF prog is attached, memory_allocated should not be changed,
> >>>>>>> but we allow a small error (up to 10 pages) in case other processes
> >>>>>>> on the host use some amounts of TCP/UDP memory.
> >>>>>>>
> >>>>>>> At 2., the test actually sends more than 1024 pages because the sysctl
> >>>>>>> net.core.mem_pcpu_rsv is 256 is by default, which means 256 pages are
> >>>>>>> buffered per cpu before reporting to sk->sk_prot->memory_allocated.
> >>>>>>>
> >>>>>>>      BUF_SINGLE (1024) * NR_SEND (64) * NR_SOCKETS (64) / 4096
> >>>>>>>      = 1024 pages
> >>>>>>>
> >>>>>>> When I reduced it to 512 pages, the following assertion for the
> >>>>>>> non-isolated case got flaky.
> >>>>>>>
> >>>>>>>      ASSERT_GT(memory_allocated[1], memory_allocated[0] + 256, ...)
> >>>>>>>
> >>>>>>> Another contributor to slowness is 150ms sleep to make sure 1 RCU
> >>>>>>> grace period passes because UDP recv queue is destroyed after that.
> >>>>>>
> >>>>>> There is a kern_sync_rcu() in testing_helpers.c.
> >>>>>
> >>>>> Nice helper :)  Will use it.
> >>>>>
> >>>>>>
> >>>>>>>
> >>>>>>>      # time ./test_progs -t sk_memcg
> >>>>>>>      #370/1   sk_memcg/TCP       :OK
> >>>>>>>      #370/2   sk_memcg/UDP       :OK
> >>>>>>>      #370/3   sk_memcg/TCPv6     :OK
> >>>>>>>      #370/4   sk_memcg/UDPv6     :OK
> >>>>>>>      #370     sk_memcg:OK
> >>>>>>>      Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED
> >>>>>>>
> >>>>>>>      real       0m1.214s
> >>>>>>>      user       0m0.014s
> >>>>>>>      sys        0m0.318s
> >>>>>>
> >>>>>> Thanks. It finished much faster in my setup also comparing with the earlier
> >>>>>> revision. However, it is a bit flaky when I run it in a loop:
> >>>>>>
> >>>>>> check_isolated:FAIL:not isolated unexpected not isolated: actual 861 <= expected 861
> >>>>>>
> >>>>>> I usually can hit this at ~40-th iteration.
> >>>>>
> >>>>> Oh.. I tested ~10 times manually but will try in a tight loop.
> >>>>
> >>>> This didn't reproduce on my QEMU with/without --enable-kvm.
> >>>>
> >>>> Changing the assert from _GT to _GE will address the very case
> >>>> above, but I'm not sure if it's enough.
> >>>
> >>> I doubled NR_SEND and it was still faster with kern_sync_rcu()
> >>> than usleep(), so I'll simply double NR_SEND in v5
> >>>
> >>> # time ./test_progs -t sk_memcg
> >>> ...
> >>> Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED
> >>> real 0m0.483s
> >>> user 0m0.010s
> >>> sys 0m0.191s
> >>>
> >>>
> >>>>
> >>>> Does the bpf CI run tests repeatedly or is this only a manual
> >>>> scenario ?
> >>
> >> I haven't seen bpf CI hit it yet. It is in my manual bash while loop. It should
> >> not be dismissed so easily. Some flaky CI tests were eventually reproduced in a
> >> loop before and fixed. I kept the bash loop continue this time until grep-ed a
> >> "0" from the error output:
> >>
> >> check_isolated:FAIL:not isolated unexpected not isolated: actual 0 <= expected 256
> >>
> >> The "long memory_allocated[2]" read from /proc/net/protocols are printed as 0
> >> but it is probably actually negative:
> >>
> >> static inline long
> >> proto_memory_allocated(const struct proto *prot)
> >> {
> >>           return max(0L, atomic_long_read(prot->memory_allocated));
> >> }
> >>
> >> prot->memory_allocated could be negative afaict but printed as 0 in
> >> /proc/net/protocols. Even the machine is network quiet after test_progs started,
> >> the "prot->memory_allocated" and the "proto->per_cpu_fw_alloc" could be in some
> >> random states before the test_progs start.  When I hit "0", it will take some
> >> efforts to send some random traffic to the machine to get the test working again. :(
> >>
> >> Also, after reading the selftest closer, I am not sure I understand why "+ 256".
> >> The "proto-> per_cpu_fw_alloc" can start with -255 or +255.
> >
> > Actually I didn't expect the random state and assumed the test's
> > local communication would complete on the same CPU thus 0~255.
> >
> > Do you see the flakiness with net.core.mem_pcpu_rsv=0 ?
> >
> > The per-cpu cache is just for performance and I think it's not
> > critical for testing and it's fine to set it to 0 during the test.
> >
> >
> >>
> >> I don't think changing NR_SEND help here. It needs a better way. May be some
> >> functions can be traced such that prot->memory_allocated can be read directly?
> >> If fentry and fexit of that function has different memory_allocated values, then
> >> the test could also become more straight forward.
> >
> > Maybe like this ?  Not yet tested, but we could attach a prog to
> > sock_init_data() or somewhere else and trigger it by additional socket(2).
> >
> >          memory_allocated = sk->sk_prot->memory_allocated;
> >          nr_cpu = bpf_num_possible_cpus();
> >
> >          for (i = 0; i < nr_cpu; i++) {
> >                  per_cpu_fw_alloc =
> > bpf_per_cpu_ptr(sk->sk_prot->per_cpu_fw_alloc, i);
>
> I suspect passing per_cpu_fw_alloc to bpf_per_cpu_ptr won't work for now. sk is
> trusted if it is a "tp_btf" but I don't think the verifier recognizes the
> sk->sk_prot is a trusted ptr. I haven't tested it though. If the above does not
> work, try to directly use the global percpu tcp_memory_per_cpu_fw_alloc. Take a
> look at how "bpf_prog_active" is used in test_ksyms_btf.c.

Thanks for the pointer !
I was looking for a way to read global vars.

>
> >                  if (per_cpu_fw_alloc)
> >                          memory_allocated += *per_cpu_fw_alloc;
>
> Yeah. I think figuring out the true memory_allocated value and use it as the
> before/after value should be good enough. Then no need to worry about the
> initial states. I wonder why proto_memory_allocated() does not do that for
> /proc/net/protocols but I guess it may not be accurate for a lot of cores.

Probably, and per_cpu_fw_alloc is expected to flip quickly.

>
> >          }
> >
> > per_cpu_fw_alloc might have been added to sk_prot->memory_allocated
> > during loop, so it's not 100% accurate still.
> >
> > Probably we should set net.core.mem_pcpu_rsv=0 and stress
> > memory_allocated before the actual test to drain per_cpu_fw_alloc
> > (at least on the testing CPU).
> I think the best is if a suitable kernel func can be traced or figure out the
> true memory_allocated value. At least figuring out the true memory_allocated
> seems doable. If nothing of the above works out, mem_pcpu_rsv=0 and
> pre-stress/pre-flush should help by getting the per_cpu_fw_alloc and
> memory_allocated to some certain states before using it in the before/after result.
>
> [ Before re-spinning, need to conclude/resolve the on-going discussion in v5 first ]

I'll wait for Shakeel's response for v5.

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