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

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Date:	Tue, 12 Feb 2008 09:41:46 -0200
From:	"Carlos R. Mafra" <crmafra@...il.com>
To:	Eric Piel <E.A.B.Piel@...elft.nl>, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [2.6.25-rc1] Strange regression with CONFIG_HZ_300=y

Eric Piel wrote:
> 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...

Nice!

> 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)

Yes, I have tickless enabled.

> See you,
> Eric

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