linux-kernel - Re: [PATCH v1 4/8] tracing/bpf: guard syscall probe with preempt

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Message-ID: <2d841991-5cae-4de4-9f10-2b65d1b0715e@efficios.com>
Date: Thu, 3 Oct 2024 20:30:13 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>,
 Alexei Starovoitov <ast@...nel.org>, Yonghong Song <yhs@...com>,
 "Paul E . McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Namhyung Kim <namhyung@...nel.org>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf <bpf@...r.kernel.org>,
 Joel Fernandes <joel@...lfernandes.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [PATCH v1 4/8] tracing/bpf: guard syscall probe with
 preempt_notrace

On 2024-10-04 01:05, Alexei Starovoitov wrote:
> On Thu, Oct 3, 2024 at 3:25 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>> On Thu,  3 Oct 2024 11:16:34 -0400
>> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>>
>>> In preparation for allowing system call enter/exit instrumentation to
>>> handle page faults, make sure that bpf can handle this change by
>>> explicitly disabling preemption within the bpf system call tracepoint
>>> probes to respect the current expectations within bpf tracing code.
>>>
>>> This change does not yet allow bpf to take page faults per se within its
>>> probe, but allows its existing probes to adapt to the upcoming change.
>>>
>>
>> I guess the BPF folks should state if this is needed or not?
>>
>> Does the BPF hooks into the tracepoints expect preemption to be disabled
>> when called?
> 
> Andrii pointed it out already.
> bpf doesn't need preemption to be disabled.
> Only migration needs to be disabled.

I'm well aware of this. Feel free to relax those constraints in
follow up patches in your own tracers. I'm simply not introducing
any behavior change in the "big switch" patch introducing faultable
syscall tracepoints. It's just too easy to overlook a dependency on
preempt off deep inside some tracer code for me to make assumptions
at the tracepoint level.

If a regression happens, it will be caused by the tracer-specific
patch that relaxes the constraints, not by the tracepoint change
that affects multiple tracers at once.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com