Message-ID: <CAAVpQUCLpi+6w1SP=FKVaXwdDHQC_P6B1hzzDC5y4brsf3_UnQ@mail.gmail.com>
Date: Thu, 4 Sep 2025 09:45:17 -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 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);
                if (per_cpu_fw_alloc)
                        memory_allocated += *per_cpu_fw_alloc;
        }

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).

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