Messages in this thread | | | Date | Wed, 27 Apr 2022 11:34:58 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH] sched/tracing: append prev_state to tp args instead |
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On 04/26/22 08:54, Andrii Nakryiko wrote: > On Tue, Apr 26, 2022 at 7:10 AM Qais Yousef <qais.yousef@arm.com> wrote: > > > > On 04/26/22 14:28, Peter Zijlstra wrote: > > > On Fri, Apr 22, 2022 at 11:30:12AM -0700, Alexei Starovoitov wrote: > > > > On Fri, Apr 22, 2022 at 10:22 AM Delyan Kratunov <delyank@fb.com> wrote: > > > > > > > > > > On Fri, 2022-04-22 at 13:09 +0200, Peter Zijlstra wrote: > > > > > > And on the other hand; those users need to be fixed anyway, right? > > > > > > Accessing prev->__state is equally broken. > > > > > > > > > > The users that access prev->__state would most likely have to be fixed, for sure. > > > > > > > > > > However, not all users access prev->__state. `offcputime` for example just takes a > > > > > stack trace and associates it with the switched out task. This kind of user > > > > > would continue working with the proposed patch. > > > > > > > > > > > If bpf wants to ride on them, it needs to suffer the pain of doing so. > > > > > > > > > > Sure, I'm just advocating for a fairly trivial patch to avoid some of the suffering, > > > > > hopefully without being a burden to development. If that's not the case, then it's a > > > > > clear no-go. > > > > > > > > > > > > Namhyung just sent this patch set: > > > > https://patchwork.kernel.org/project/netdevbpf/patch/20220422053401.208207-3-namhyung@kernel.org/ > > > > > > That has: > > > > > > + * recently task_struct->state renamed to __state so it made an incompatible > > > + * change. > > > > > > git tells me: > > > > > > 2f064a59a11f ("sched: Change task_struct::state") > > > > > > is almost a year old by now. That don't qualify as recently in my book. > > > That says that 'old kernels used to call this...'. > > > > > > > to add off-cpu profiling to perf. > > > > It also hooks into sched_switch tracepoint. > > > > Notice it deals with state->__state rename just fine. > > > > > > So I don't speak BPF much; it always takes me more time to make bpf work > > > than to just hack up the kernel, which makes it hard to get motivated. > > > > > > However, it was not just a rename, state changed type too, which is why I > > > did the rename, to make sure all users would get a compile fail and > > > could adjust. > > > > > > If you're silently making it work by frobbing the name, you loose that. > > > > > > Specifically, task_struct::state used to be 'volatile long', while > > > task_struct::__state is 'unsigned int'. As such, any user must now be > > > very careful to use READ_ONCE(). I don't see that happening with just > > > frobbing the name. > > > > > > Additinoally, by shrinking the field, I suppose BE systems get to keep > > > the pieces? > > > > > > > But it will have a hard time without this patch > > > > until we add all the extra CO-RE features to detect > > > > and automatically adjust bpf progs when tracepoint > > > > arguments order changed. > > > > > > Could be me, but silently making it work sounds like fail :/ There's a > > > reason code changes, users need to adapt, not silently pretend stuff is > > > as before. > > > > > > How will you know you need to fix your tool? > > > > If libbpf doesn't fail, then yeah it's a big problem. I wonder how users of > > kprobe who I suppose are more prone to this kind of problems have been coping. > > See my reply to Peter. libbpf can't know user's intent to fail this > automatically, in general. In some cases when it can it does > accommodate this automatically. In other cases it provides instruments > for user to handle this (bpf_core_field_size(), > BPF_CORE_READ_BITFIELD(), etc).
My naiive thinking is that the function signature has changed (there's 1 extra arg not just a subtle swap of args of the same type) - so I thought that can be detected. But maybe it is harder said than done.
I am trying to remember as I've used this before; I think you get the arg list as part of ctx when you attach to a function?
I wonder if it'd be hard to provide a macro for the user to provide the signature of the function they expect; this macro can try then to verify/assert the number, type and order is the same. Not bullet proof and requires opt-in, but could be useful?
// dummy pseudo-code
BPF_CORE_ASSERT_SIG(sched_switch, NR_ARGS, ARG0, ARG1, ...) if (ctx->nr_args != NR_ARGS) assert() if (type_of(ctx->args[0]) != type_of(ARG0)) assert() ...
I'm not sure if you have any info about the type though..
> But in the end no one eliminated the need for testing your application > for correctness. Tracing programs do break on kernel changes and BPF > users do adapt to them. Sometimes adapting is easy (like state -> > __state transition), sometimes it's much more involved (like this > argument order change).
It's not just an arg re-order, it's a new argument inserted in the middle. But fair enough :-)
Cheers
-- Qais Yousef
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