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From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     rostedt@goodmis.org, bristot@kernel.org, paulmck@kernel.org
Cc:     mingo@redhat.com, linux-kernel@vger.kernel.org,
        mtosatti@redhat.com, Nicolas Saenz Julienne <nsaenzju@redhat.com>
Subject: [PATCH] tracing/osnoise: Force quiescent states while tracing
Date:   Mon, 28 Feb 2022 15:14:23 +0100
Message-Id: <20220228141423.259691-1-nsaenzju@redhat.com>
MIME-Version: 1.0
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At the moment running osnoise on an isolated CPU and a PREEMPT_RCU
kernel might have the side effect of extending grace periods too much.
This will eventually entice RCU to schedule a task on the isolated CPU
to end the overly extended grace period, adding unwarranted noise to the
CPU being traced in the process.

So, check if we're the only ones running on this isolated CPU and that
we're on a PREEMPT_RCU setup. If so, let's force quiescent states in
between measurements.

Non-PREEMPT_RCU setups don't need to worry about this as osnoise main
loop's cond_resched() will go though a quiescent state for them.

Note that this same exact problem is what extended quiescent states were
created for. But adapting them to this specific use-case isn't trivial
as it'll imply reworking entry/exit and dynticks/context tracking code.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
---
 kernel/trace/trace_osnoise.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 870a08da5b48..4928358f6e88 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -21,7 +21,9 @@
 #include <linux/uaccess.h>
 #include <linux/cpumask.h>
 #include <linux/delay.h>
+#include <linux/tick.h>
 #include <linux/sched/clock.h>
+#include <linux/sched/isolation.h>
 #include <uapi/linux/sched/types.h>
 #include <linux/sched.h>
 #include "trace.h"
@@ -1295,6 +1297,7 @@ static int run_osnoise(void)
 	struct osnoise_sample s;
 	unsigned int threshold;
 	u64 runtime, stop_in;
+	unsigned long flags;
 	u64 sum_noise = 0;
 	int hw_count = 0;
 	int ret = -1;
@@ -1386,6 +1389,22 @@ static int run_osnoise(void)
 					osnoise_stop_tracing();
 		}
 
+		/*
+		 * Check if we're the only ones running on this nohz_full CPU
+		 * and that we're on a PREEMPT_RCU setup. If so, let's fake a
+		 * QS since there is no way for RCU to know we're not making
+		 * use of it.
+		 *
+		 * Otherwise it'll be done through cond_resched().
+		 */
+		if (IS_ENABLED(CONFIG_PREEMPT_RCU) &&
+		    !housekeeping_cpu(raw_smp_processor_id(), HK_FLAG_MISC) &&
+		    tick_nohz_tick_stopped()) {
+			local_irq_save(flags);
+			rcu_momentary_dyntick_idle();
+			local_irq_restore(flags);
+		}
+
 		/*
 		 * For the non-preemptive kernel config: let threads runs, if
 		 * they so wish.
-- 
2.35.1