Re: [patch] CFS scheduler, -v8

From: Esben Nielsen
Date: Sat May 05 2007 - 04:32:20 EST




On Wed, 2 May 2007, Ingo Molnar wrote:


* Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

The problem is with comparing a s64 values with (s64)ULONG_MAX, which
evaluates to -1. Then we check if exec_delta64 and fair_delta64 are
greater than (s64)ULONG_MAX (-1), if so we assign (s64)ULONG_MAX to
the respective values.

ah, indeed ...

The fix is to compare these values against (s64)LONG_MAX and assign
(s64)LONG_MAX to exec_delta64 and fair_delta64 if they are greater
than (s64)LONG_MAX.

Tested on PowerPC, the regression is gone, tasks are load balanced as
they were in v7.

thanks, applied!

Ingo

I have been wondering why you use usigned for timers anyway. It is also like that in hrtimers. Why not use signed and avoid (almost) all worries about wrap around issues. The trick is that when all
a < b
is be replaced by
a - b < 0
the code will work on all 2-complement machines even if the (signed!) integers a and b wrap around.

In both the hrtimer and CFS patch 32 bit timers could be used internally on 32 bit architectures to avoid expensive 64 bit integer calculations.
The only thing is: timeouts can not be bigger than 2^31 in the chosen units.

I have successfully coded a (much more primitive) hrtimer system for another OS on both ARM and PPC using the above trick in my former job. On both I used the machine's internal clock as the internal representation of time and I only scaled to a struct timespec in the user interface.

Esben
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