Xen devel - Jul 2006 - [PATCH] xentrace: new trace event to track lost trace records

Simple addition to trace.c to count trace records that are generated 
when the trace buffers are full. When there is space available again, a 
new record is generated that indicates how many trace records have been 
thrown away. I''m working on the corresponding userland code to do 
something useful with this information, and will send out an additional 
patch ''soon.''

Rob Gardner

# HG changeset patch
# User rob.gardner@hp.com
# Node ID 81e7d6e0f9109bf493b7c8281ffc79d10f4dd2ab
# Parent  67f658b84025efeb2a570d2937ebc8f5e35056f7

New trace event is introduced to track lost trace records.

diff -r 67f658b84025 -r 81e7d6e0f910 xen/common/trace.c
--- a/xen/common/trace.c    Thu Jul  6 18:08:37 2006
+++ b/xen/common/trace.c    Thu Jul  6 23:29:26 2006
@@ -234,7 +234,8 @@
     struct t_buf *buf;
     struct t_rec *rec;
     unsigned long flags;
-
+    static long lost_records = 0;
+   
     BUG_ON(!tb_init_done);
 
     if ( (tb_event_mask & event) == 0 )
@@ -261,8 +262,24 @@
 
     if ( (buf->prod - buf->cons) >= nr_recs )
     {
+        lost_records++;
         local_irq_restore(flags);
         return;
+    }
+
+    if (lost_records) {
+        rec = &t_recs[smp_processor_id()][buf->prod % nr_recs];
+        rec->cycles  = (u64)get_cycles();
+        rec->event   = TRC_LOST_RECORDS;
+        rec->data[0] = lost_records;
+        rec->data[1] = 0;
+        rec->data[2] = 0;
+        rec->data[3] = 0;
+        rec->data[4] = 0;
+       
+        wmb();
+        buf->prod++;
+        lost_records = 0;
     }
 
     rec = &t_recs[smp_processor_id()][buf->prod % nr_recs];
diff -r 67f658b84025 -r 81e7d6e0f910 xen/include/public/trace.h
--- a/xen/include/public/trace.h    Thu Jul  6 18:08:37 2006
+++ b/xen/include/public/trace.h    Thu Jul  6 23:29:26 2006
@@ -26,6 +26,7 @@
 #define TRC_VMXIO    0x00088000   /* VMX io emulation trace  */
 
 /* Trace events per class */
+#define TRC_LOST_RECORDS        (TRC_GEN + 1)
 
 #define TRC_SCHED_DOM_ADD       (TRC_SCHED +  1)
 #define TRC_SCHED_DOM_REM       (TRC_SCHED +  2)



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I like this idea.  Thanks for all the work you''re doing on the trace
buffer.
> diff -r 67f658b84025 -r 81e7d6e0f910 xen/common/trace.c
> --- a/xen/common/trace.c    Thu Jul  6 18:08:37 2006
> +++ b/xen/common/trace.c    Thu Jul  6 23:29:26 2006
> @@ -234,7 +234,8 @@
>      struct t_buf *buf;
>      struct t_rec *rec;
>      unsigned long flags;
> -
> +    static long lost_records = 0;
> +
Shouldn''t this be a per-CPU counter?  Otherwise there are going to be
issues
with multiple CPUs accessing it, and with not knowing which CPU lost the 
data.

Although I guess we''re very unlikely to ever overflow this, perhaps
it''s also
worth considering making it a saturating counter so that userland knows the 
count may not be accurate?  Just a random thought...
> +    if (lost_records) {
> +        rec = &t_recs[smp_processor_id()][buf->prod % nr_recs];
You could use &this_cpu[t_recs] here - I think the rest of the code is now 
converted to use this format.

Cheers,
Mark
> +        rec->cycles  = (u64)get_cycles();
> +        rec->event   = TRC_LOST_RECORDS;
> +        rec->data[0] = lost_records;
> +        rec->data[1] = 0;
> +        rec->data[2] = 0;
> +        rec->data[3] = 0;
> +        rec->data[4] = 0;
> +
> +        wmb();
> +        buf->prod++;
> +        lost_records = 0;
>      }
>
>      rec = &t_recs[smp_processor_id()][buf->prod % nr_recs];
> diff -r 67f658b84025 -r 81e7d6e0f910 xen/include/public/trace.h
> --- a/xen/include/public/trace.h    Thu Jul  6 18:08:37 2006
> +++ b/xen/include/public/trace.h    Thu Jul  6 23:29:26 2006
> @@ -26,6 +26,7 @@
>  #define TRC_VMXIO    0x00088000   /* VMX io emulation trace  */
>
>  /* Trace events per class */
> +#define TRC_LOST_RECORDS        (TRC_GEN + 1)
>
>  #define TRC_SCHED_DOM_ADD       (TRC_SCHED +  1)
>  #define TRC_SCHED_DOM_REM       (TRC_SCHED +  2)
-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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