linux-kernel - [PATCH V4 08/12] trace/hwlat: Remove printk from sampling loop

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Date:   Tue, 15 Jun 2021 11:28:47 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Phil Auld <pauld@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kate Carcia <kcarcia@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Clark Willaims <williams@...hat.com>,
        John Kacur <jkacur@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH V4 08/12] trace/hwlat: Remove printk from sampling loop

hwlat has some time operation checks on the sample loop, and it is
currently using pr_err (printk) to report them. The problem is that
this can lead the system to an unresponsible state due to an overflow of
printk messages. This problem can be mitigated by writing the error
message to the trace buffer.

Remove the printk messages from the sampling loop, switching the to
messages in the trace buffer.

No functional change.

Cc: Jonathan Corbet <corbet@....net>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Alexandre Chartre <alexandre.chartre@...cle.com>
Cc: Clark Willaims <williams@...hat.com>
Cc: John Kacur <jkacur@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org
Cc: linux-doc@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
---
 kernel/trace/trace_hwlat.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 0ab6acf6ae9a..86e8541d4331 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -182,6 +182,15 @@ void trace_hwlat_callback(bool enter)
 		kdata->nmi_count++;
 }
 
+/*
+ * hwlat_err - report a hwlat error.
+ */
+#define hwlat_err(msg) ({							\
+	struct trace_array *tr = hwlat_trace;					\
+										\
+	trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_, msg);	\
+})
+
 /**
  * get_sample - sample the CPU TSC and look for likely hardware latencies
  *
@@ -225,7 +234,7 @@ static int get_sample(void)
 			outer_diff = time_to_us(time_sub(t1, last_t2));
 			/* This shouldn't happen */
 			if (outer_diff < 0) {
-				pr_err(BANNER "time running backwards\n");
+				hwlat_err(BANNER "time running backwards\n");
 				goto out;
 			}
 			if (outer_diff > outer_sample)
@@ -237,7 +246,7 @@ static int get_sample(void)
 
 		/* Check for possible overflows */
 		if (total < last_total) {
-			pr_err("Time total overflowed\n");
+			hwlat_err("Time total overflowed\n");
 			break;
 		}
 		last_total = total;
@@ -253,7 +262,7 @@ static int get_sample(void)
 
 		/* This shouldn't happen */
 		if (diff < 0) {
-			pr_err(BANNER "time running backwards\n");
+			hwlat_err(BANNER "time running backwards\n");
 			goto out;
 		}
 
-- 
2.31.1