[RFC PATCH 5/5] x86,smp: add debugging code to track spinlock delay value

[Rik van Riel <riel@redhat.com>]

From: Eric Dumazet <eric.dumazet@gmail.com>

This code prints out the maximum spinlock delay value and the
backtrace that pushed it that far.

On systems with serial consoles, the act of printing can cause
the spinlock delay value to explode. It can still be useful as
a debugging tool, but is probably too verbose to merge upstream
in this form.

Not-signed-off-by: Rik van Riel <riel@redhat.com>
Not-signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 arch/x86/kernel/smp.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 29360c4..a4401ed 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -148,6 +148,8 @@ static DEFINE_PER_CPU(struct delay_entry [1 << DELAY_HASH_SHIFT], spinlock_delay
 	},
 };
 
+static DEFINE_PER_CPU(u16, maxdelay);
+
 void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 {
 	__ticket_t head = inc.head, ticket = inc.tail;
@@ -195,6 +197,12 @@ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 	}
 	ent->hash = hash;
 	ent->delay = delay;
+
+	if (__this_cpu_read(maxdelay) < delay) {
+		pr_err("cpu %d lock %p delay %d\n", smp_processor_id(), lock, delay);
+		__this_cpu_write(maxdelay, delay);
+		WARN_ON(1);
+	}
 }
 
 /*