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authorSteven Rostedt <srostedt@redhat.com>2012-11-02 18:33:05 -0400
committerSteven Rostedt <rostedt@goodmis.org>2013-01-22 23:38:03 -0500
commit567cd4da54ff45513d2ca1f0e3cb9ba45b66d6cf (patch)
tree1a9e719a31643138fa76ecf556401fee1f536813 /kernel/trace/ring_buffer.c
parent897f68a48b1f8fb6cb7493e1ee37e3ed7f879937 (diff)
ring-buffer: User context bit recursion checking
Using context bit recursion checking, we can help increase the performance of the ring buffer. Before this patch: # echo function > /debug/tracing/current_tracer # for i in `seq 10`; do ./hackbench 50; done Time: 10.285 Time: 10.407 Time: 10.243 Time: 10.372 Time: 10.380 Time: 10.198 Time: 10.272 Time: 10.354 Time: 10.248 Time: 10.253 (average: 10.3012) Now we have: # echo function > /debug/tracing/current_tracer # for i in `seq 10`; do ./hackbench 50; done Time: 9.712 Time: 9.824 Time: 9.861 Time: 9.827 Time: 9.962 Time: 9.905 Time: 9.886 Time: 10.088 Time: 9.861 Time: 9.834 (average: 9.876) a 4% savings! Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'kernel/trace/ring_buffer.c')
-rw-r--r--kernel/trace/ring_buffer.c85
1 files changed, 60 insertions, 25 deletions
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 6ff9cc4658ed..481e26269281 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2432,41 +2432,76 @@ rb_reserve_next_event(struct ring_buffer *buffer,
#ifdef CONFIG_TRACING
-#define TRACE_RECURSIVE_DEPTH 16
+/*
+ * The lock and unlock are done within a preempt disable section.
+ * The current_context per_cpu variable can only be modified
+ * by the current task between lock and unlock. But it can
+ * be modified more than once via an interrupt. To pass this
+ * information from the lock to the unlock without having to
+ * access the 'in_interrupt()' functions again (which do show
+ * a bit of overhead in something as critical as function tracing,
+ * we use a bitmask trick.
+ *
+ * bit 0 = NMI context
+ * bit 1 = IRQ context
+ * bit 2 = SoftIRQ context
+ * bit 3 = normal context.
+ *
+ * This works because this is the order of contexts that can
+ * preempt other contexts. A SoftIRQ never preempts an IRQ
+ * context.
+ *
+ * When the context is determined, the corresponding bit is
+ * checked and set (if it was set, then a recursion of that context
+ * happened).
+ *
+ * On unlock, we need to clear this bit. To do so, just subtract
+ * 1 from the current_context and AND it to itself.
+ *
+ * (binary)
+ * 101 - 1 = 100
+ * 101 & 100 = 100 (clearing bit zero)
+ *
+ * 1010 - 1 = 1001
+ * 1010 & 1001 = 1000 (clearing bit 1)
+ *
+ * The least significant bit can be cleared this way, and it
+ * just so happens that it is the same bit corresponding to
+ * the current context.
+ */
+static DEFINE_PER_CPU(unsigned int, current_context);
-/* Keep this code out of the fast path cache */
-static noinline void trace_recursive_fail(void)
+static __always_inline int trace_recursive_lock(void)
{
- /* Disable all tracing before we do anything else */
- tracing_off_permanent();
-
- printk_once(KERN_WARNING "Tracing recursion: depth[%ld]:"
- "HC[%lu]:SC[%lu]:NMI[%lu]\n",
- trace_recursion_buffer(),
- hardirq_count() >> HARDIRQ_SHIFT,
- softirq_count() >> SOFTIRQ_SHIFT,
- in_nmi());
+ unsigned int val = this_cpu_read(current_context);
+ int bit;
- WARN_ON_ONCE(1);
-}
-
-static inline int trace_recursive_lock(void)
-{
- trace_recursion_inc();
+ if (in_interrupt()) {
+ if (in_nmi())
+ bit = 0;
+ else if (in_irq())
+ bit = 1;
+ else
+ bit = 2;
+ } else
+ bit = 3;
- if (likely(trace_recursion_buffer() < TRACE_RECURSIVE_DEPTH))
- return 0;
+ if (unlikely(val & (1 << bit)))
+ return 1;
- trace_recursive_fail();
+ val |= (1 << bit);
+ this_cpu_write(current_context, val);
- return -1;
+ return 0;
}
-static inline void trace_recursive_unlock(void)
+static __always_inline void trace_recursive_unlock(void)
{
- WARN_ON_ONCE(!trace_recursion_buffer());
+ unsigned int val = this_cpu_read(current_context);
- trace_recursion_dec();
+ val--;
+ val &= this_cpu_read(current_context);
+ this_cpu_write(current_context, val);
}
#else