linux-kernel - [PATCH] cleanup sched_yield (sys)call nesting.

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Message-Id: <1258577194.12429.86.camel@sven.thebigcorporation.com>
Date:	Wed, 18 Nov 2009 12:46:34 -0800
From:	Sven-Thorsten Dietrich <sven@...bigcorporation.com>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Leon Woestenberg <leon.woestenberg@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-i2c@...r.kernel.org,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] cleanup sched_yield (sys)call nesting.

On Wed, 2009-11-18 at 17:52 +0100, Jean Delvare wrote:
> On Wed, 18 Nov 2009 17:28:53 +0100, Leon Woestenberg wrote:
> > On Wed, Nov 18, 2009 at 2:05 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > > Our timers are very efficient and some day we will need to make jiffies a
> > > function and stop the timer ticking for best performance. At that point
> > > timers are probably the most efficient way to do much of this.
> >
> > The problem with I2C bitbanged is the stringent timing, we need a way
> > to have fine-grained sleeping
> > mixed with real-time tasks in order to make this work.
> 
> FWIW, the problem that was initially reported has nothing to do with
> this. i2c-algo-bit used mdelay() during transactions, not yield().
> yield() is used only in once place, _between_ transactions attempts.
> There are no strict timing constraints there.
> 

I agree that dropping out sched_yield entirely should maybe start by
deprecating / flagging as a warning in sched_rt.c.

This is just a minimal cleanup I stumbled across while looking at it -
to get away from the uglyness of calling into the syscall interface from
inside the Kernel.

I'll generate something more substantial for discussion later.

Subject: clean up chaining in sched_yield()
From: Sven-Thorsten Dietrich <sdietrich@...e.de>

The call to sys_sched_yield for in-Kernel is messy.
and the return code from sys_sched_yield is ignored when called from
in-kernel.

Signed-off-by: Sven-Thorsten Dietrich <sdietrich@...e.de>

diff --git a/kernel/sched.c b/kernel/sched.c
index 3c11ae0..db2c0f9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6647,12 +6647,12 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
 }
 
 /**
- * sys_sched_yield - yield the current processor to other threads.
+ * do_sched_yield - yield the current processor to other threads.
  *
  * This function yields the current CPU to other tasks. If there are no
  * other threads running on this CPU then this function will return.
  */
-SYSCALL_DEFINE0(sched_yield)
+static inline void do_sched_yield(void) 
 {
 	struct rq *rq = this_rq_lock();
 
@@ -6669,6 +6669,11 @@ SYSCALL_DEFINE0(sched_yield)
 	preempt_enable_no_resched();
 
 	schedule();
+}
+
+SYSCALL_DEFINE0(sched_yield)
+{
+	do_sched_yield();
 
 	return 0;
 }
@@ -6746,7 +6751,7 @@ EXPORT_SYMBOL(__cond_resched_softirq);
 void __sched yield(void)
 {
 	set_current_state(TASK_RUNNING);
-	sys_sched_yield();
+	do_sched_yield();
 }
 EXPORT_SYMBOL(yield);
 


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