[RFC][PATCH 07/10] x86: Remove unlikey()s from sched_switch segment tests

Steven Rostedt
2010-12-07T02:14:03Z

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From: Steven Rostedt <srostedt@redhat.com>

On a 64bit distro, the chances of having a process using segment registers
is very unlikely. But if the userspace is 32bit running on top of
a 64bit kernel (very common), then this will be very likely that
processes have segment registers in use.

Running on my main desktop (which is a 32bit userspace on top of
a 64bit kernel) the annotated branch profiler showed the following:

correct incorrect % Function File Line
------- --------- - -------- ---- ----
25522442 304125815 92 __switch_to process_64.c 408
25522430 304123341 92 __switch_to process_64.c 412
25743877 303891250 92 __switch_to process_64.c 464

Instead of punishing 32bit userspace systems with an unlikely, just remove
the unlikely and let gcc optimize for what it thinks is good and let
the branch prediction (hopefully) work.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
arch/x86/kernel/process_64.c | 6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index b3d7a3a..22de90c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -405,11 +405,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
* This won't pick up thread selector changes, but I guess that is ok.
*/
savesegment(es, prev->es);
- if (unlikely(next->es | prev->es))
+ if (next->es | prev->es)
loadsegment(es, next->es);

savesegment(ds, prev->ds);
- if (unlikely(next->ds | prev->ds))
+ if (next->ds | prev->ds)
loadsegment(ds, next->ds);

@@ -461,7 +461,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
wrmsrl(MSR_FS_BASE, next->fs);
prev->fsindex = fsindex;

- if (unlikely(gsindex | next->gsindex | prev->gs)) {
+ if (gsindex | next->gsindex | prev->gs) {
load_gs_index(next->gsindex);
if (gsindex)
prev->gs = 0;
--
1.7.2.3