linux-kernel - [PATCH 2/3] tracing: Use in_serving_softirq() to deduct softirq status.

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Date:   Wed, 13 Jan 2021 00:00:56 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/3] tracing: Use in_serving_softirq() to deduct softirq status.

PREEMPT_RT does not report "serving softirq" because the tracing core
looks at the preemption counter while PREEMPT_RT does not update it
while processing softirqs in order to remain preemptible. The
information is stored somewhere else.
The in_serving_softirq() macro and the SOFTIRQ_OFFSET define are still
working but not on the preempt-counter.

Use in_serving_softirq() macro which works on PREEMPT_RT. On !PREEMPT_RT
the compiler (gcc-10 / clang-11) is smart enough to optimize the
in_serving_softirq() related read of the preemption counter away.
The only difference I noticed by using in_serving_softirq() on
!PREEMPT_RT is that gcc-10 implemented tracing_gen_ctx_flags() as
reading FLAG, jmp _tracing_gen_ctx_flags(). Without in_serving_softirq()
it inlined _tracing_gen_ctx_flags() into tracing_gen_ctx_flags().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/trace/trace.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 679112d3b3da9..416b63e05960e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2596,8 +2596,7 @@ unsigned int _tracing_gen_ctx_flags(unsigned long irqflags)
 		trace_flags |= TRACE_FLAG_NMI;
 	if (pc & HARDIRQ_MASK)
 		trace_flags |= TRACE_FLAG_HARDIRQ;
-
-	if (pc & SOFTIRQ_OFFSET)
+	if (in_serving_softirq())
 		trace_flags |= TRACE_FLAG_SOFTIRQ;

 	if (tif_need_resched())
-- 
2.30.0