linux-kernel - Re: [2.6.25-rc1] Strange regression with CONFIG_HZ

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Date:	Tue, 12 Feb 2008 09:26:28 +0100
From:	Eric Piel <E.A.B.Piel@...elft.nl>
To:	"Carlos R. Mafra" <crmafra@...il.com>
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [2.6.25-rc1] Strange regression with CONFIG_HZ_300=y

Carlos R. Mafra wrote:
> 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@...or.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

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