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authorSteven Rostedt <srostedt@redhat.com>2011-06-24 23:28:13 -0400
committerSteven Rostedt <rostedt@goodmis.org>2011-07-07 22:39:38 -0400
commit4376cac66778b25e599be3f5d54f33f58ba8ead7 (patch)
tree9cf05820586f140bafcde67cd2cd80a4391ef7eb /kernel/trace
parente4a3f541f0b67fdad98b326c851dfe7f4b6b6dad (diff)
ftrace: Do not disable interrupts for modules in mcount update
When I mounted an NFS directory, it caused several modules to be loaded. At the time I was running the preemptirqsoff tracer, and it showed the following output: # tracer: preemptirqsoff # # preemptirqsoff latency trace v1.1.5 on 2.6.33.9-rt30-mrg-test # -------------------------------------------------------------------- # latency: 1177 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4) # ----------------- # | task: modprobe-19370 (uid:0 nice:0 policy:0 rt_prio:0) # ----------------- # => started at: ftrace_module_notify # => ended at: ftrace_module_notify # # # _------=> CPU# # / _-----=> irqs-off # | / _----=> need-resched # || / _---=> hardirq/softirq # ||| / _--=> preempt-depth # |||| /_--=> lock-depth # |||||/ delay # cmd pid |||||| time | caller # \ / |||||| \ | / modprobe-19370 3d.... 0us!: ftrace_process_locs <-ftrace_module_notify modprobe-19370 3d.... 1176us : ftrace_process_locs <-ftrace_module_notify modprobe-19370 3d.... 1178us : trace_hardirqs_on <-ftrace_module_notify modprobe-19370 3d.... 1178us : <stack trace> => ftrace_process_locs => ftrace_module_notify => notifier_call_chain => __blocking_notifier_call_chain => blocking_notifier_call_chain => sys_init_module => system_call_fastpath That's over 1ms that interrupts are disabled on a Real-Time kernel! Looking at the cause (being the ftrace author helped), I found that the interrupts are disabled before the code modification of mcounts into nops. The interrupts only need to be disabled on start up around this code, not when modules are being loaded. Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'kernel/trace')
-rw-r--r--kernel/trace/ftrace.c16
1 files changed, 11 insertions, 5 deletions
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index c997f7371c65..df93392aad89 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3318,7 +3318,7 @@ static int ftrace_process_locs(struct module *mod,
{
unsigned long *p;
unsigned long addr;
- unsigned long flags;
+ unsigned long flags = 0; /* Shut up gcc */
mutex_lock(&ftrace_lock);
p = start;
@@ -3336,12 +3336,18 @@ static int ftrace_process_locs(struct module *mod,
}
/*
- * Disable interrupts to prevent interrupts from executing
- * code that is being modified.
+ * We only need to disable interrupts on start up
+ * because we are modifying code that an interrupt
+ * may execute, and the modification is not atomic.
+ * But for modules, nothing runs the code we modify
+ * until we are finished with it, and there's no
+ * reason to cause large interrupt latencies while we do it.
*/
- local_irq_save(flags);
+ if (!mod)
+ local_irq_save(flags);
ftrace_update_code(mod);
- local_irq_restore(flags);
+ if (!mod)
+ local_irq_restore(flags);
mutex_unlock(&ftrace_lock);
return 0;