Re: [PATCH v4 bpf-next/net 5/5] selftest: bpf: Add test for SK_BPF_MEMCG_SOCK_ISOLATED.

On 9/4/25 9:45 AM, Kuniyuki Iwashima wrote:

On Wed, Sep 3, 2025 at 10:51 PM Martin KaFai Lau [off-list ref] wrote:

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On 9/3/25 10:08 AM, Kuniyuki Iwashima wrote:

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On Wed, Sep 3, 2025 at 9:59 AM Kuniyuki Iwashima [off-list ref] wrote:

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On Tue, Sep 2, 2025 at 1:49 PM Kuniyuki Iwashima [off-list ref] wrote:

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On Tue, Sep 2, 2025 at 1:26 PM Martin KaFai Lau [off-list ref] wrote:

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On 8/28/25 6:00 PM, Kuniyuki Iwashima wrote:

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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.

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     # 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

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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.

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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.

                 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.

         }

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 ]