perf sched: Fix bad event alignment

perf sched: Fix bad event alignment

perf sched raises the following error when it meets a sched
switch event:

perf: builtin-sched.c:286: register_pid: Assertion `!(pid >= 65536)' failed.
Abandon

Currently in x86-64, the sched switch events have a hole in the
middle of the structure:

	u16 common_type;
	u8 common_flags;
	u8 common_preempt_count;
	u32 common_pid;
	u32 common_tgid;

	char prev_comm[16];
	u32 prev_pid;
	u32 prev_prio;
			<--- there
	u64 prev_state;
	char next_comm[16];
	u32 next_pid;
	u32 next_prio;

Gcc inserts a 4 bytes hole there for prev_state to be u64
aligned. And the events are exported to userspace with this
hole.

But in userspace, from perf sched, we fetch it using a
structure that has a new field in the beginning: u32 size. This
is because our trace is exported with its size as a field. But
now that we have this new field, the hole in the middle
disappears because it makes prev_state becoming well aligned.

And since we are using a pointer to the raw trace using this
struct, instead of reading prev_state, we are reading the hole.

We could fix it by keeping the size seperate from the struct
but actually there a lot of other potential problems: some
fields may be saved as long in a 64 bits system and later read
as long in a 32 bits system. Also this direct cast doesn't care
about the endianness differences between the host traced
machine and the machine in which we do the post processing.

So instead of using such dangerous direct casts, fetch the
values using the trace parsing API that already takes care of
all these problems.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>