Messages in this thread | | | Date | Tue, 12 Feb 2008 09:26:28 +0100 | From | Eric Piel <> | Subject | Re: [2.6.25-rc1] Strange regression with CONFIG_HZ_300=y |
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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@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)
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