[PATCH 3/9] ftrace: Optimize testing what context current is in

Steven Rostedt
2020-10-29T00:55:57Z

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

The preempt_count() is not a simple location in memory, it could be part of
per_cpu code or more. Each access to preempt_count(), or one of its accessor
functions (like in_interrupt()) takes several cycles. By reading
preempt_count() once, and then doing tests to find the context against the
value return is slightly faster than using in_nmi() and in_interrupt().

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
include/linux/trace_recursion.h | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index da17a1144bf4..04d21e9ec48b 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -111,22 +111,29 @@ enum {

#define TRACE_CONTEXT_MASK TRACE_LIST_MAX

+/*
+ * Used for setting context
+ * NMI = 0
+ * IRQ = 1
+ * SOFTIRQ = 2
+ * NORMAL = 3
+ */
+enum {
+ TRACE_CTX_NMI,
+ TRACE_CTX_IRQ,
+ TRACE_CTX_SOFTIRQ,
+ TRACE_CTX_NORMAL,
+};
+
static __always_inline int trace_get_context_bit(void)
{
- int bit;
-
- if (in_interrupt()) {
- if (in_nmi())
- bit = 0;
+ unsigned long pc = preempt_count();

- else if (in_irq())
- bit = 1;
- else
- bit = 2;
- } else
- bit = 3;
-
- return bit;
+ if (!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
+ return TRACE_CTX_NORMAL;
+ else
+ return pc & NMI_MASK ? TRACE_CTX_NMI :
+ pc & HARDIRQ_MASK ? TRACE_CTX_IRQ : TRACE_CTX_SOFTIRQ;
}

static __always_inline int trace_test_and_set_recursion(int start, int max)
--
2.28.0