Re: slow UML on x86-64, soft lockups

[Paolo Giarrusso <blaisorblade@yahoo.it>]

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Jeff Garzik <jeff@garzik.org> ha scritto:

> Recent 2.6.18 / 2.6.19-rc kernels run at the expected speed, on
> 32-bit 
> x86, with a Fedora Core 5 or 6 UML userland.  However, on 64-bit
> x86-64 
> with a 64-bit UML userland, the kernel is achingly slow.  It works,
> 
> but...  Login takes several minutes (via ssh from host or xterm 
> console), and soft lockup traces continually print to the screen
> (see 
> output below).  Once logged into, programs work, but again, very
> slowly. 
>   Commands which complete in under a second normally often takes
> minutes.
> 

> Any ideas?  I guess there is a bug in the 64-bit UML timer code?
Yes, there is. Jeff, after our discussion, did I send you the patch
for this (not sure whether tested, I haven't reproduced this bug yet,
but fairly obvious)?

It is attached but I'm answering from a webmail (it happens since I'm
so busy), so I decline responsibility for any MIME content (I'm
sorry, yes).

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Index: linux-2.6.git/arch/um/sys-i386/delay.c
===================================================================
--- linux-2.6.git.orig/arch/um/sys-i386/delay.c
+++ linux-2.6.git/arch/um/sys-i386/delay.c
@@ -27,14 +27,3 @@ void __udelay(unsigned long usecs)
 }
 
 EXPORT_SYMBOL(__udelay);
-
-void __const_udelay(unsigned long usecs)
-{
-	int i, n;
-
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
-}
-
-EXPORT_SYMBOL(__const_udelay);
Index: linux-2.6.git/arch/um/sys-x86_64/delay.c
===================================================================
--- linux-2.6.git.orig/arch/um/sys-x86_64/delay.c
+++ linux-2.6.git/arch/um/sys-x86_64/delay.c
@@ -28,14 +28,3 @@ void __udelay(unsigned long usecs)
 }
 
 EXPORT_SYMBOL(__udelay);
-
-void __const_udelay(unsigned long usecs)
-{
-	unsigned long i, n;
-
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
-}
-
-EXPORT_SYMBOL(__const_udelay);
Index: linux-2.6.git/include/asm-um/delay.h
===================================================================
--- linux-2.6.git.orig/include/asm-um/delay.h
+++ linux-2.6.git/include/asm-um/delay.h
@@ -1,9 +1,20 @@
 #ifndef __UM_DELAY_H
 #define __UM_DELAY_H
 
-#include "asm/arch/delay.h"
-#include "asm/archparam.h"
-
 #define MILLION 1000000
 
+/* Undefined on purpose */
+extern void __bad_udelay(void);
+extern void __bad_ndelay(void);
+
+extern void __udelay(unsigned long usecs);
+extern void __const_udelay(unsigned long usecs);
+extern void __delay(unsigned long loops);
+
+#define udelay(n) ((__builtin_constant_p(n) && (n) > 20000) ? \
+	__bad_udelay() : __udelay(n))
+
+/* It appears that ndelay is not used at all for UML. */
+#define ndelay(n) __bad_ndelay()
+
 #endif