tracing: Remove the extra 4 bytes of padding in events

Due to a userspace issue with PowerTop v2beta, which hardcoded
the offset of event fields that it was using, it broke when
we removed the Big Kernel Lock counter from the event header.

 (commit e6e1e2593 "tracing: Remove lock_depth from event entry")

Because this broke userspace, it was determined that we must
keep those 4 bytes around.

 (commit a3a4a5acd "Regression: partial revert "tracing: Remove lock_depth from event entry"")

This unfortunately wastes space in the ring buffer. 4 bytes per
event, where a lot of events are just 24 bytes. That's 16% of the
buffer wasted. A million events will add 4 megs of white space
into the buffer.

It was later noticed that PowerTop v2beta could not work on systems
where the kernel was 64 bit but the userspace was 32 bits.
The reason was because the offsets are different between the
two and the hard coded offset of one would not work with the other.

With PowerTop v2 final, it implemented the same interface that both
perf and trace-cmd use. That is, it reads the format file of
the event to find the offsets of the fields it needs. This fixes
the problem with running powertop on a 32 bit userspace running
on a 64 bit kernel. It also no longer requires the 4 byte padding.

As PowerTop v2 has been out for a while, and is included in all
major distributions, it is time that we can safely remove the
4 bytes of padding. Users of PowerTop v2beta should upgrade to
PowerTop v2 final.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 43ef8b6cce7fb26c2f1c2f8e4c16da112d4a9fec..6f8d0b77006b203df54d4afeb488a6a2df67c522 100644 (file)
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -49,7 +49,6 @@ struct trace_entry {
        unsigned char           flags;
        unsigned char           preempt_count;
        int                     pid;
-       int                     padding;
 };
 
 #define FTRACE_MAX_EVENT                                               \

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d62248dfda760ce8b91fba6a192a29121c0f9b34..a387bd271e711484670b855d0fc5deac643f4743 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1173,7 +1173,6 @@ tracing_generic_entry_update(struct trace_entry *entry, unsigned long flags,
 
        entry->preempt_count            = pc & 0xff;
        entry->pid                      = (tsk) ? tsk->pid : 0;
-       entry->padding                  = 0;
        entry->flags =
 #ifdef CONFIG_TRACE_IRQFLAGS_SUPPORT
                (irqs_disabled_flags(flags) ? TRACE_FLAG_IRQS_OFF : 0) |

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 880073d0b946fc58865b426511b038ca24271004..57e9b284250c4ea30d7ee11692066644099acc32 100644 (file)
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -116,7 +116,6 @@ static int trace_define_common_fields(void)
        __common_field(unsigned char, flags);
        __common_field(unsigned char, preempt_count);
        __common_field(int, pid);
-       __common_field(int, padding);
 
        return ret;
 }