linux-kernel - [RFC PATCH 1/2] Use pause-on-trace with the latency tracers

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Message-ID: <20210119164344.37500-2-Viktor.Rosendahl@bmw.de>
Date:   Tue, 19 Jan 2021 17:43:43 +0100
From:   Viktor Rosendahl <Viktor.Rosendahl@....de>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        <linux-kernel@...r.kernel.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        Viktor Rosendahl <Viktor.Rosendahl@....de>
Subject: [RFC PATCH 1/2] Use pause-on-trace with the latency tracers

Eaerlier, tracing was disabled when reading the trace file. This behavior
was changed with:

commit 06e0a548bad0 ("tracing: Do not disable tracing when reading the
trace file").

This doesn't seem to work with the latency tracers.

The above mentioned commit dit not only change the behavior but also added
an option to emulate the old behavior. The idea with this patch is to
enable this pause-on-trace option when the latency tracers are used.

This is a workaround, perhaps it would be better to make the latency
tracers work without pausing but I am not sure how to do that, or even
how feasible it is without significant rework.

Signed-off-by: Viktor Rosendahl <Viktor.Rosendahl@....de>
---
 kernel/trace/trace_irqsoff.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index d06aab4dcbb8..6756379b661f 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -562,6 +562,8 @@ static int __irqsoff_tracer_init(struct trace_array *tr)
 	/* non overwrite screws up the latency tracers */
 	set_tracer_flag(tr, TRACE_ITER_OVERWRITE, 1);
 	set_tracer_flag(tr, TRACE_ITER_LATENCY_FMT, 1);
+	/* without pause, we will produce garbage if another latency occurs */
+	set_tracer_flag(tr, TRACE_ITER_PAUSE_ON_TRACE, 1);
 
 	tr->max_latency = 0;
 	irqsoff_trace = tr;
@@ -583,11 +585,13 @@ static void __irqsoff_tracer_reset(struct trace_array *tr)
 {
 	int lat_flag = save_flags & TRACE_ITER_LATENCY_FMT;
 	int overwrite_flag = save_flags & TRACE_ITER_OVERWRITE;
+	int pause_flag = save_flags & TRACE_ITER_PAUSE_ON_TRACE;
 
 	stop_irqsoff_tracer(tr, is_graph(tr));
 
 	set_tracer_flag(tr, TRACE_ITER_LATENCY_FMT, lat_flag);
 	set_tracer_flag(tr, TRACE_ITER_OVERWRITE, overwrite_flag);
+	set_tracer_flag(tr, TRACE_ITER_PAUSE_ON_TRACE, pause_flag);
 	ftrace_reset_array_ops(tr);
 
 	irqsoff_busy = false;
-- 
2.25.1