perf/x86: Warn to early_printk() in case irq_work is too slow

On Mon, Feb 10, 2014 at 08:45:16AM -0800, Dave Hansen wrote:
> The reason I coded this up was that NMIs were firing off so fast that
> nothing else was getting a chance to run.  With this patch, at least the
> printk() would come out and I'd have some idea what was going on.

It will start spewing to early_printk() (which is a lot nicer to use
from NMI context too) when it fails to queue the IRQ-work because its
already enqueued.

It does have the false-positive for when two CPUs trigger the warn
concurrently, but that should be rare and some extra clutter on the
early printk shouldn't be a problem.

Cc: hpa@zytor.com
Cc: tglx@linutronix.de
Cc: dzickus@redhat.com
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: mingo@kernel.org
Fixes: 6a02ad66b2c4 ("perf/x86: Push the duration-logging printk() to IRQ context")
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140211150116.GO27965@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2067cbb378eb..45e5543e2a1e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -243,7 +243,7 @@ static void perf_duration_warn(struct irq_work *w)
         printk_ratelimited(KERN_WARNING
                         "perf interrupt took too long (%lld > %lld), lowering "
                         "kernel.perf_event_max_sample_rate to %d\n",
-                        avg_local_sample_len, allowed_ns,
+                        avg_local_sample_len, allowed_ns >> 1,
                         sysctl_perf_event_sample_rate);
 }
 
@@ -283,7 +283,12 @@ void perf_sample_event_took(u64 sample_len_ns)
 
         update_perf_cpu_limits();
 
-        irq_work_queue(&perf_duration_work);
+        if (!irq_work_queue(&perf_duration_work)) {
+                early_printk("perf interrupt took too long (%lld > %lld), lowering "
+                             "kernel.perf_event_max_sample_rate to %d\n",
+                             avg_local_sample_len, allowed_ns >> 1,
+                             sysctl_perf_event_sample_rate);
+        }
 }
 
 static atomic64_t perf_event_id;