[Bug 26982] New: drm-next/linux-next glxgears CPU usage doubles for 32bit, doesn't on 64bit

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=26982

           Summary: drm-next/linux-next glxgears CPU usage doubles for
                    32bit, doesn't on 64bit
           Product: Drivers
           Version: 2.5
    Kernel Version: 2.6.37-rc7-next-20101228+
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
        AssignedTo: drivers_video-dri at kernel-bugs.osdl.org
        ReportedBy: bitbytebit at gmail.com
        Regression: Yes


Somewhere in the drm-next tree and linux-next, between December 13th (
2.6.37-rc5-next-20101213+)and 27th (2.6.37-rc7-next-20101228+), a change made
the CPU usage on a 32bit system when running any application utilizing the
DRM/Vblank/page flipping (testing on ATI Radeon 4350 card at least) including
glxgears.  This doesn't happen on 64 bit, it still uses the same amount of CPU
as before but the 32bit version of the kernel jumped almost double usage.  This
can be seen in Mame using Mesa/DRI and page flipping (classic), but glxgears
shows it although it's not as magnified there since it's a very low cpu usage
app anyways.  To show the scale, basically an app that used to take 80% cpu
usage on one core now requires 130-150% cpu usage and needs another core now.  

I suspect, since looking at changes between those dates, the only real change
which could do this  seems like the vblank timestamp stuff added in during that
time.  I have tried just the drm-next code or DRM code itself lifted from these
kernels and put into 2.6.36.2 and it still does it, so doesn't have anything to
do with the other parts of the kernel.  It seems to be centered in the
DRM/Radeon code somewhere.  

So to recreate it should be easy, taking the newest linux-next version and
running and benchmarking glxgears or any vblank using application (using the
newest Mesa and DRI, X Windows from Git and xf86-ati driver from pageflipping
branch).  Doing this on a 32bit build and comparing to a 64bit build, it should
be pretty noticeable, it is here at least.  I am using Gentoo with the newest X
Windows GIT overlay, and linux-next of course.  This is on a core2duo system
from HP, an xw4400 workstation, radeon 4350 card using KMS and page
flipping/vblank.

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