Carlos R. Mafra wrote:
quoted text > I apologize in advance if I am crazy about this, but I noticed
> a strange regression wrt 2.6.24 in cpufreq (I think) in 2.6.25-rc1, which
> goes away if I revert the following commit:
>
> commit bdc807871d58285737d50dc6163d0feb72cb0dc2
> Author: H. Peter Anvin <hpa@zytor.com>
> Date: Fri Feb 8 04:21:26 2008 -0800
>
> avoid overflows in kernel/time.c
>
> When the conversion factor between jiffies and milli- or microseconds is
> not a single multiply or divide, as for the case of HZ == 300, we currently
> do a multiply followed by a divide. The intervening result, however, is
> subject to overflows, especially since the fraction is not simplified (for
> HZ == 300, we multiply by 300 and divide by 1000).
>
> This is exposed to the user when passing a large timeout to poll(), for
> example.
>
> This patch replaces the multiply-divide with a reciprocal multiplication on
> 32-bit platforms. When the input is an unsigned long, there is no portable
> way to do this on 64-bit platforms there is no portable way to do this
> since it requires a 128-bit intermediate result (which gcc does support on
> 64-bit platforms but may generate libgcc calls, e.g. on 64-bit s390), but
> since the output is a 32-bit integer in the cases affected, just simplify
> the multiply-divide (*3/10 instead of *300/1000).
>
> The reciprocal multiply used can have off-by-one errors in the upper half
> of the valid output range. This could be avoided at the expense of having
> to deal with a potential 65-bit intermediate result. Since the intent is
> to avoid overflow problems and most of the other time conversions are only
> semiexact, the off-by-one errors were considered an acceptable tradeoff.
>
> [...]
> [more text follows]
>
> The problem in vanilla 2.6.25-rc1 happens with CONFIG_HZ_300=y (and doesn't
> with CONFIG_HZ_250=y or with the above commit reverted). The cpu frequency doesn't
> change anymore regardless of the load, and it stays high (2.0 GHz or 1.2 GHz) even
> when idle (I checked with 'top'), when the usual is to go to 800 Mhz when idle (I
> always use the ondemand governor compiled in and as the default governor).
>
> The laptop is a Vaio VGN-FZ240E, core 2 duo T7250 @ 2.0 GHz and the kernel is x86_64.
Hi, it's great you found out the culprit commit because I was really
wondering where this bug was coming from...
As a data point, my machine has a core 2 duo @ 1.2GHz and x86_64 arch.
Do you also have the tickless option activated? (it could play a role)
See you,
Eric
