Re: [PATCH] sched/tracing: append prev_state to tp args instead

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Delyan Kratunov <delyank@fb.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: "peterz@infradead.org" <peterz@infradead.org>,
	"bigeasy@linutronix.de" <bigeasy@linutronix.de>,
	"dietmar.eggemann@arm.com" <dietmar.eggemann@arm.com>,
	"keescook@chromium.org" <keescook@chromium.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"andrii@kernel.org" <andrii@kernel.org>,
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Subject: Re: [PATCH] sched/tracing: append prev_state to tp args instead
Date: Fri, 22 Apr 2022 11:30:12 -0700	[thread overview]
Message-ID: <CAADnVQKsah4aka-LJ+X+5XHHESKbbw36D8fXTLqcYp2io3PN_w@mail.gmail.com> (raw)
In-Reply-To: <056e9bb0d0e3fc20572d42db7386face1d0665d6.camel@fb.com>

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/

to add off-cpu profiling to perf.
It also hooks into sched_switch tracepoint.
Notice it deals with state->__state rename just fine.
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.
We will do it eventually, of course.
There will be additional work in llvm, libbpf, kernel, etc.
But for now I think it would be good to land Delyan's patch
to avoid unnecessary pain to all the users.

Peter, do you mind?