Re: dynamic-hz

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Dec 13, 2004 at 03:02:37AM -0800, Andrew Morton wrote:
> We should retain the option of compile-time constant HZ - it's
> easy enough.  Probably the patch already does that.

The patch only does HZ at dynamic time. But of course it's absolutely
trivial to define it at compile time, it's probably a 3 liner on top of
my current patch ;). However personally I don't think the three liner
will worth the few seconds more spent configuring the kernel ;).

The HZ cacheline is pure readonly (actually I'm not defining it as
cacheline_aligned, I probably should, __HZ can go together with
__SHIFT_HZ). The only debug option I introduced (because it could have a
performance penalty) is a check that nobody ever attempts to read HZ
before we initialized it by parsing the boot command line. If that
happens I printk and then I fallback to the fixed-HZ, so machine works
fine even in case of bugs and I get the debugging printk. That code
actually never triggered once. I did it primarly during development to
be sure I could debug fast troubles with other archs (this is already
running in all archs with SLES8).

This is pretty much the core of the patch:

+extern unsigned long __HZ;
+
+static inline unsigned long get_hz(void)
+{
+#ifdef CONFIG_DEBUG_HZ
+	if (unlikely(!__HZ)) {
+		__label__ here;
+		printk("early HZ: %p\n", &&here);
+	here:
+		init_HZ(USER_HZ);
+	}
+#endif /* CONFIG_DEBUG_HZ */
+	return __HZ;
+}
+
+#define HZ get_hz()
+
+#define CLOCKS_PER_SEC	(USER_HZ)	/* like times() */
+
+#define jiffies_to_clock_t(x)	(likely((HZ) >= (USER_HZ)) ?
\
+				 (x + ((HZ) / (USER_HZ)) - 1) / ((HZ) /
(USER_HZ)) :	\
+				 (x) * ((USER_HZ) / (HZ)))
+#define user_to_kernel_hz(x)	(likely((HZ) >= (USER_HZ)) ?
\
+				 (x) * ((HZ) / (USER_HZ)) :
\
+				 (x + ((USER_HZ) / (HZ)) - 1) /
((USER_HZ) / (HZ)))
+#define user_to_kernel_hz_overflow(x)	((x * (HZ) + (USER_HZ) - 1) / (USER_HZ))

[..]

+++ x/kernel/sched.c	2004-05-31 15:51:42.722918448 +0200
@@ -45,6 +45,8 @@
 #define TASK_USER_PRIO(p)	USER_PRIO((p)->static_prio)
 #define MAX_USER_PRIO		(USER_PRIO(MAX_PRIO))
 
+unsigned long __HZ, __SHIFT_HZ;
+
 /*