RFC: time_adj for HZ==250 in kernel/timer.c ?

[Harald Koenig]

Hello linux-kernel, hi Ulrich,

first: as I'm not subscribed to linix-kernel list, please send answers with CC: PM.  thanks!


playing with the openSUSE 10.0 beta-test I slipped over problems with procinfo etc.
so I noticed that at least suse/novell again changed the scheduler frequency HZ
from 1000 to 250.  which led me to the following question,  for which I got no
commets from SUSE so far, and since it's not a suse-only issue anyway... 
 

the linux kernel for suse 10.0 will run with HZ==250 by default (9.x used HZ==1000).

        harald:~ # cat /proc/version
        Linux version 2.6.13-8-default (geeko@buildhost) (gcc version 4.0.2 20050901 (prerelease) (SUSE Linux)) #1 Tue Sep 6 12:59:22 UTC 2005

for HZ==100 and HZ==1000 there is special code in kernel/timer.c to 
adjust the value of time_adj  because HZ isn't a power of two and
clock stuff uses some binary shift operations instead of divides
(and 100!=128 or 1000!=1024).

what about having a similar special case for the new HZ==250 too ??
using 250 instead of 256 (SHIFT_HZ==8), which is an error of 2.4 %,
identical to 1000 vs. 1024.

any feelings or comments about the following patch ?

-------------------------------------------------------------------------------
--- kernel/timer.c~     2005-08-22 22:50:27.000000000 +0200
+++ kernel/timer.c      2005-08-31 15:12:57.801644750 +0200
@@ -752,7 +752,7 @@
     else
        time_adj += (time_adj >> 2) + (time_adj >> 5);
 #endif
-#if HZ == 1000
+#if HZ == 1000 || HZ == 250
     /* Compensate for (HZ==1000) != (1 << SHIFT_HZ).
      * Add 1.5625% and 0.78125% to get 1023.4375; => only 0.05% error (p. 14)
      */
-------------------------------------------------------------------------------


or maybe (just in case...:) even

#if HZ == 1000 || HZ == 500 || HZ == 250

most likely this specific compensation should be correct 
for any integet multiple of 125, isn't it ?

similarly, the "HZ == 100" compensation should be ok for HZ==25/50/200/400/800 too
according to include/linux/jiffies.h.


comments ?

Harald
-- 
"I hope to die                                      ___       _____
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                                                    \  \/OOOOOOOOOOOOOOO\
                                                      \ OOOOOOOOOOOOOOOOO|//
Harald Koenig,                                         \/\/\/\/\/\/\/\/\/
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