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authorSteven Rostedt <srostedt@redhat.com>2009-12-11 11:54:51 -0500
committerSteven Rostedt <rostedt@goodmis.org>2009-12-11 13:19:51 -0500
commitcc51a0fca66658ea710db566ba17e80e3f7d4957 (patch)
tree4ab424a7f5f873ba603d090ab36b15f5aaf874c3 /kernel/trace
parent03889384cee7a198a79447c1ea6aca2c8e54d155 (diff)
tracing: Add stack trace to irqsoff tracer
The irqsoff and friends tracers help in finding causes of latency in the kernel. The also work with the function tracer to show what was happening when interrupts or preemption are disabled. But the function tracer has a bit of an overhead and can cause exagerated readings. Currently, when tracing with /proc/sys/kernel/ftrace_enabled = 0, where the function tracer is disabled, the information that is provided can end up being useless. For example, a 2 and a half millisecond latency only showed: # tracer: preemptirqsoff # # preemptirqsoff latency trace v1.1.5 on 2.6.32 # -------------------------------------------------------------------- # latency: 2463 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) # ----------------- # | task: -4242 (uid:0 nice:0 policy:0 rt_prio:0) # ----------------- # => started at: _spin_lock_irqsave # => ended at: remove_wait_queue # # # _------=> CPU# # / _-----=> irqs-off # | / _----=> need-resched # || / _---=> hardirq/softirq # ||| / _--=> preempt-depth # |||| /_--=> lock-depth # |||||/ delay # cmd pid |||||| time | caller # \ / |||||| \ | / hackbenc-4242 2d.... 0us!: trace_hardirqs_off <-_spin_lock_irqsave hackbenc-4242 2...1. 2463us+: _spin_unlock_irqrestore <-remove_wait_queue hackbenc-4242 2...1. 2466us : trace_preempt_on <-remove_wait_queue The above lets us know that hackbench with pid 2463 grabbed a spin lock somewhere and enabled preemption at remove_wait_queue. This helps a little but where this actually happened is not informative. This patch adds the stack dump to the end of the irqsoff tracer. This provides the following output: hackbenc-4242 2d.... 0us!: trace_hardirqs_off <-_spin_lock_irqsave hackbenc-4242 2...1. 2463us+: _spin_unlock_irqrestore <-remove_wait_queue hackbenc-4242 2...1. 2466us : trace_preempt_on <-remove_wait_queue hackbenc-4242 2...1. 2467us : <stack trace> => sub_preempt_count => _spin_unlock_irqrestore => remove_wait_queue => free_poll_entry => poll_freewait => do_sys_poll => sys_poll => system_call_fastpath Now we see that the culprit of this latency was the free_poll_entry code. Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'kernel/trace')
-rw-r--r--kernel/trace/trace_irqsoff.c2
1 files changed, 2 insertions, 0 deletions
diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 3aa7eaa2114..2974bc7538c 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -151,6 +151,8 @@ check_critical_timing(struct trace_array *tr,
goto out_unlock;
trace_function(tr, CALLER_ADDR0, parent_ip, flags, pc);
+ /* Skip 5 functions to get to the irq/preempt enable function */
+ __trace_stack(tr, flags, 5, pc);
if (data->critical_sequence != max_sequence)
goto out_unlock;