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    SubjectRe: [patch] i386: use thread_info flags for debug regs and IO bitmaps
    In-Reply-To: <Pine.LNX.4.64.0607081425430.3869@g5.osdl.org>

    On Sat, 8 Jul 2006 14:26:53 -0700, Linus Torvalds wrote:
    >
    > On Fri, 7 Jul 2006, Chuck Ebbert wrote:
    > >
    > > From: Stephane Eranian <eranian@hpl.hp.com>
    > >
    > > Use thread info flags to track use of debug registers and IO bitmaps.
    > >
    > > - add TIF_DEBUG to track when debug registers are active
    > > - add TIF_IO_BITMAP to track when I/O bitmap is used
    > > - modify __switch_to() to use the new TIF flags
    >
    > Can you explain what the advantages of this are?

    Stephane's perfmon2 patch adds yet another special-case to the
    switch_to() code, so Andi suggested this change. It will allow
    the perfmon2 patch to have no performance impact on normal
    task-switching, since it will just use another flag.

    After I saw a ~7% gain in task-switch performance, I like it now
    even without perfmon2 in there.

    > I don't see it. It's just creating new state to describe state that we
    > already had, and as far as I can tell, it's just a way to potentially have
    > more new bugs thanks to the new state getting out of sync with the old
    > one?

    Well yeah, there is that. But Andi and I both reviewed it and he's
    already put the x86_64 version into his tree. Testing in -mm should
    show whether there are any problems.

    --
    Chuck
    "You can't read a newspaper if you can't read." --George W. Bush
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