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

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Message-ID: <CAADnVQJf535hwud5XtQKStOge9=pYVYWSiq_8Q2YAvN5rba==A@mail.gmail.com>
Date: Thu, 3 Oct 2024 16:05:33 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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 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.