From b000c8065a92b0fe0e1694f41b2c8d8ba7b7b1ec Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Fri, 18 Jan 2013 10:31:20 -0500 Subject: [PATCH] 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 Acked-by: Arjan van de Ven Signed-off-by: Steven Rostedt --- include/linux/ftrace_event.h | 1 - kernel/trace/trace.c | 1 - kernel/trace/trace_events.c | 1 - 3 files changed, 3 deletions(-) diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h index 43ef8b6cce7f..6f8d0b77006b 100644 --- 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 d62248dfda76..a387bd271e71 100644 --- 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 880073d0b946..57e9b284250c 100644 --- 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; } -- 2.30.2