Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG_PARAVIRT

[Waiman Long <llong@redhat.com>]

On 1/8/25 10:53 PM, Alexei Starovoitov wrote:
> On Wed, Jan 8, 2025 at 7:46 PM Waiman Long <llong@redhat.com> wrote:
>>>>>> As for the locking semantics allowed by the BPF verifier, is it possible
>>>>>> to enforce the strict locking rules for PREEMPT_RT kernel and use the
>>>>>> relaxed semantics for non-PREEMPT_RT kernel. We don't want the loading
>>>>>> of an arbitrary BPF program to break the latency guarantee of a
>>>>>> PREEMPT_RT kernel.
>>>>> Not really.
>>>>> root can load silly bpf progs that take significant
>>>>> amount time without abusing spinlocks.
>>>>> Like 100k integer divides or a sequence of thousands of calls to map_update.
>>>>> Long runtime of broken progs is a known issue.
>>>>> We're working on a runtime termination check/watchdog that
>>>>> will detect long running progs and will terminate them.
>>>>> Safe termination is tricky, as you can imagine.
>>>> Right.
>>>>
>>>> In that case, we just have to warn users that they can load BPF prog at
>>>> their own risk and PREEMPT_RT kernel may break its latency guarantee.
>>> Let's not open this can of worms.
>>> There will be a proper watchdog eventually.
>>> If we start to warn, when do we warn? On any bpf program loaded?
>>> How about classic BPF ? tcpdump and seccomp ? They are limited
>>> to 4k instructions, but folks can abuse that too.
>> My intention is to document this somewhere, not to print out a warning
>> in the kernel dmesg log.
> Document what exactly?
> "Loading arbitrary BPF program may break the latency guarantee of PREEMPT_RT"
> ?
> That's not helpful to anyone.
> Especially it undermines the giant effort we did together
> with RT folks to make bpf behave well on RT.
> For a long time bpf was the only user of migrate_disable().
> Some of XDP bits got friendly to RT only in the last release. Etc.

OK, it is just a suggestion. If you don't think that is necessary, I am 
not going to insist. Anyway, users should thoroughly test their BPF 
program before deplolying on production systems.

Cheers,
Longman