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    SubjectRe: hackbench regression since 2.6.25-rc
    On Fri, Mar 14, 2008 at 08:50:19AM +0800, Zhang, Yanmin wrote:
    > On Thu, 2008-03-13 at 10:12 -0700, Greg KH wrote:
    > > On Thu, Mar 13, 2008 at 09:19:21AM -0700, Randy Dunlap wrote:
    > > > On Thu, 13 Mar 2008 08:14:13 -0700 Greg KH wrote:
    > > >
    > > > > On Thu, Mar 13, 2008 at 03:46:57PM +0800, Zhang, Yanmin wrote:
    > > > > > Comparing with 2.6.24, on my 16-core tigerton, hackbench process mode has about
    > > > > > 40% regression with 2.6.25-rc1, and more than 20% regression with kernel
    > > > > > 2.6.25-rc4, because rc4 includes the reverting patch of scheduler load balance.
    > > > > >
    > > > > > Command to start it.
    > > > > > #hackbench 100 process 2000
    > > > > > I ran it for 3 times and sum the values.
    > > > > >
    > > > > > I tried to investiagte it by bisect.
    > > > > > Kernel up to tag 0f4dafc0563c6c49e17fe14b3f5f356e4c4b8806 has the 20% regression.
    > > > > > Kernel up to tag 6e90aa972dda8ef86155eefcdbdc8d34165b9f39 hasn't regression.
    > > > > >
    > > > > > Any bisect between above 2 tags cause kernel hang. I tried to checkout to a point between
    > > > > > these 2 tags for many times manually and kernel always paniced.
    > > > >
    > > > > Where is the kernel panicing? The changeset right after the last one
    > > > > above: bc87d2fe7a1190f1c257af8a91fc490b1ee35954, is a change to efivars,
    > > > > are you using that in your .config?
    > > > >
    > > > > > All patches between the 2 tags are on kobject restructure. I guess such restructure
    > > > > > creates more cache miss on the 16-core tigerton.
    > > > >
    > > > > Nothing should be creating kobjects on a normal load like this, so a
    > > > > regression seems very odd. Unless the /sys/kernel/uids/ stuff is
    > > > > triggering this?
    > > > >
    > > > > Do you have a link to where I can get hackbench (google seems to find
    > > > > lots of reports with it, but not the source itself), so I can test to
    > > > > see if we are accidentally creating kobjects with this load?
    > > >
    > > > The version that I see referenced most often (unscientifically :)
    > > > is somewhere under people.redhat.com/mingo/, like so:
    > > > http://people.redhat.com/mingo/cfs-scheduler/tools/hackbench.c
    > >
    > > Great, thanks for the link.
    > >
    > > In using that version, I do not see any kobjects being created at all
    > > when running the program. So I don't see how a kobject change could
    > > have caused any slowdown.
    > >
    > > Yanmin, is the above link the version you are using?
    > Yes.
    >
    > >
    > > Hm, running with "hackbench 100 process 2000" seems to lock up my
    > > laptop, maybe I shouldn't run 4000 tasks at once on such a memory
    > > starved machine...
    > The issue doesn't exist on my 8-core stoakley and on tulsa. So I don't think
    > you could reproduce it on laptop.

    But I should see any kobjects being created and destroyed as you are
    thinking that is the problem here, right?

    And I don't see any, so I'm thinking that this is probably something
    else.

    I'm still interested in why your machine was oopsing when bisecting
    through the kobject commits. I thought it all should have worked
    without problems, as I spend enough time trying to ensure it was so...

    thanks,

    greg k-h


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