2008-12-16 14:51:19

by Metzger, Markus T

Subject: [patch 2/3] x86, bts: remove recursion from get_context

Optimistically allocate a DS context. It is extremely unlikely that
one already existed. This simplifies the code a lot.

Signed-off-by: Markus Metzger <[email protected]>
Reported-by: Ingo Molnar <[email protected]>
---

Index: ftrace/arch/x86/kernel/ds.c
===================================================================
--- ftrace.orig/arch/x86/kernel/ds.c 2008-12-12 14:02:24.000000000 +0100
+++ ftrace/arch/x86/kernel/ds.c 2008-12-12 16:25:22.000000000 +0100
@@ -232,53 +232,45 @@

#define system_context per_cpu(system_context_array, smp_processor_id())

-static struct ds_context *ds_get_context(struct task_struct *task)
+
+static inline struct ds_context *ds_get_context(struct task_struct *task)
{
struct ds_context **p_context =
(task ? &task->thread.ds_ctx : &system_context);
- struct ds_context *context = *p_context;
+ struct ds_context *context = NULL;
+ struct ds_context *new_context = NULL;
unsigned long irq;

- if (!context) {
- context = kzalloc(sizeof(*context), GFP_KERNEL);
- if (!context)
- return NULL;
-
- spin_lock_irqsave(&ds_lock, irq);
-
- if (*p_context) {
- kfree(context);
+ /* Chances are small that we already have a context. */
+ new_context = kzalloc(sizeof(*new_context), GFP_KERNEL);
+ if (!new_context)
+ return NULL;

- context = *p_context;
- } else {
- *p_context = context;
+ spin_lock_irqsave(&ds_lock, irq);

- context->this = p_context;
- context->task = task;
+ context = *p_context;
+ if (!context) {
+ context = new_context;

- if (task)
- set_tsk_thread_flag(task, TIF_DS_AREA_MSR);
+ context->this = p_context;
+ context->task = task;
+ context->count = 0;

- if (!task || (task == current))
- wrmsrl(MSR_IA32_DS_AREA,
- (unsigned long)context->ds);
- }
+ if (task)
+ set_tsk_thread_flag(task, TIF_DS_AREA_MSR);

- context->count++;
+ if (!task || (task == current))
+ wrmsrl(MSR_IA32_DS_AREA, (unsigned long)context->ds);

- spin_unlock_irqrestore(&ds_lock, irq);
- } else {
- spin_lock_irqsave(&ds_lock, irq);
+ *p_context = context;
+ }

- context = *p_context;
- if (context)
- context->count++;
+ context->count++;

- spin_unlock_irqrestore(&ds_lock, irq);
+ spin_unlock_irqrestore(&ds_lock, irq);

- if (!context)
- context = ds_get_context(task);
- }
+ if (context != new_context)
+ kfree(new_context);

return context;
}
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2008-12-16 17:27:19

by Ingo Molnar

Subject: Re: [patch 2/3] x86, bts: remove recursion from get_context

* Markus Metzger <[email protected]> wrote:

> Optimistically allocate a DS context. It is extremely unlikely that
> one already existed. This simplifies the code a lot.
>
> Signed-off-by: Markus Metzger <[email protected]>
> Reported-by: Ingo Molnar <[email protected]>

nice cleanup - applied to tip/tracing/hw-branch-tracer - thanks Markus!

Ingo