linux-kernel - Re: [PATCH v1 3/8] tracing/perf: guard syscall probe with preempt

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Message-ID: <Zv80GJWwUVwPwvgf@pavilion.home>
Date: Fri, 4 Oct 2024 02:17:28 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	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@...r.kernel.org,
	Joel Fernandes <joel@...lfernandes.org>,
	linux-trace-kernel@...r.kernel.org,
	Michael Jeanson <mjeanson@...icios.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v1 3/8] tracing/perf: guard syscall probe with
 preempt_notrace

Le Thu, Oct 03, 2024 at 06:25:08PM -0400, Steven Rostedt a écrit :
> On Thu,  3 Oct 2024 11:16:33 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
> > In preparation for allowing system call enter/exit instrumentation to
> > handle page faults, make sure that perf can handle this change by
> > explicitly disabling preemption within the perf system call tracepoint
> > probes to respect the current expectations within perf ring buffer code.
> > 
> > This change does not yet allow perf to take page faults per se within
> > its probe, but allows its existing probes to adapt to the upcoming
> > change.
> 
> Frederic,
> 
> Does the perf ring buffer expect preemption to be disabled when used?
> 
> In other words, is this patch needed?

At least the trace events perf callback requires that because it uses
a per cpu buffer (see perf_trace_buf_alloc()).

Thanks.