linux-kernel - Re: [PATCH v2 40/40] tracing: Add trace_event_buffer

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Message-ID: <20170908162756.74c6be8a@gandalf.local.home>
Date:   Fri, 8 Sep 2017 16:27:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     tglx@...utronix.de, mhiramat@...nel.org, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel.opensrc@...il.com, joelaf@...gle.com,
        mathieu.desnoyers@...icios.com, baohong.liu@...el.com,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 40/40] tracing: Add trace_event_buffer_reserve()
 variant that allows recursion

On Tue,  5 Sep 2017 16:57:52 -0500
Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:

> Synthetic event generation requires the reservation of a second event
> while the reservation of a previous event is still in progress.  The
> trace_recursive_lock() check in ring_buffer_lock_reserve() prevents
> this however.
> 
> This sets up a special reserve pathway for this particular case,
> leaving existing pathways untouched, other than an additional check in
> ring_buffer_lock_reserve() and trace_event_buffer_reserve().  These
> checks could be gotten rid of as well, with copies of those functions,
> but for now try to avoid that unless necessary.
> 
> Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>

I've been planing on changing that lock, which may help you here
without having to mess around with parameters. That is to simply add a
counter. Would this patch help you. You can add a patch to increment
the count to 5 with an explanation of handling synthetic events, but
even getting to 4 is extremely unlikely.

I'll make this into an official patch if this works for you, and then
you can include it in your series.

-- Steve

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 0bcc53e..9dbb459 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2582,61 +2582,29 @@ rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
  * 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.
+ * be modified more than once via an interrupt. There are four
+ * different contexts that we need to consider.
  *
- *  bit 0 =  NMI context
- *  bit 1 =  IRQ context
- *  bit 2 =  SoftIRQ context
- *  bit 3 =  normal context.
+ *  Normal context.
+ *  SoftIRQ context
+ *  IRQ context
+ *  NMI 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.
+ * If for some reason the ring buffer starts to recurse, we
+ * only allow that to happen at most 4 times (one for each
+ * context). If it happens 5 times, then we consider this a
+ * recusive loop and do not let it go further.
  */
 
 static __always_inline int
 trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	unsigned int val = cpu_buffer->current_context;
-	int bit;
-
-	if (in_interrupt()) {
-		if (in_nmi())
-			bit = RB_CTX_NMI;
-		else if (in_irq())
-			bit = RB_CTX_IRQ;
-		else
-			bit = RB_CTX_SOFTIRQ;
-	} else
-		bit = RB_CTX_NORMAL;
-
-	if (unlikely(val & (1 << bit)))
+	if (cpu_buffer->current_context >= 4)
 		return 1;
 
-	val |= (1 << bit);
-	cpu_buffer->current_context = val;
+	cpu_buffer->current_context++;
+	/* Interrupts must see this update */
+	barrier();
 
 	return 0;
 }
@@ -2644,7 +2612,9 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
 static __always_inline void
 trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	cpu_buffer->current_context &= cpu_buffer->current_context - 1;
+	/* Don't let the dec leak out */
+	barrier();
+	cpu_buffer->current_context--;
 }
 
 /**