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    SubjectRe: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()
    On Thu, Oct 01, 2009 at 10:02:07AM +0800, Nick Piggin wrote:
    > On Mon, Sep 28, 2009 at 04:44:01PM +0800, Wu Fengguang wrote:
    > > On Mon, Sep 28, 2009 at 03:20:25AM +0800, Nick Piggin wrote:
    > > > On Sun, Sep 27, 2009 at 06:47:39PM +0800, Wu Fengguang wrote:
    > > > > >
    > > > > > And standard deviation is 0.04%, much larger than the difference 0.008% ..
    > > > >
    > > > > Sorry that's not correct. I improved the accounting by treating
    > > > > function0+function1 from two CPUs as an integral entity:
    > > > >
    > > > > total time add_to_page_cache_lru percent stddev
    > > > > before 3880166848.722 9683329.610 0.250% 0.014%
    > > > > after 3828516894.376 9778088.870 0.256% 0.012%
    > > > > delta 0.006%
    > > >
    > > > I don't understand why you're doing this NFS workload to measure?
    > >
    > > Because it is the first convenient workload hit my mind, and avoids
    > > real disk IO :)
    >
    > Using tmpfs or sparse files is probably a lot easier.

    Good ideas. In fact I tried them in the very beginning.
    The ratios are roughly at the same level (which is somehow unexpected):

    total time add_to_page_cache_lru percent stddev
    tmpfs 1579056274.576 2615476.234 0.1656354036338758%
    sparse 1074931917.425 3001273 0.27920586888791538%

    Workload is to copy 1G /dev/zero to /dev/shm/, or 1G sparse file
    (ext2) to /dev/null.

    echo 1 > /debug/tracing/function_profile_enabled
    cp /dev/zero /dev/shm/
    echo 0 > /debug/tracing/function_profile_enabled

    dd if=/dev/zero of=/mnt/test bs=1k count=1 seek=1048575
    echo 1 > /debug/tracing/function_profile_enabled
    cp /mnt/test/sparse /dev/null
    echo 0 > /debug/tracing/function_profile_enabled

    > > > I see significant nfs, networking protocol and device overheads in
    > > > your profiles, also you're hitting some locks or something which
    > > > is causing massive context switching. So I don't think this is a
    > > > good test.
    > >
    > > Yes there are overheads. However it is a real and common workload.
    >
    > Right, but so are lots of other workloads that don't hit
    > add_to_page_cache heavily :)
    >
    >
    > > > But anyway as Hugh points out, you need to compare with a
    > > > *completely* fixed kernel, which includes auditing all users of page
    > > > flags non-atomically (slab, notably, but possibly also other
    > > > places).
    > >
    > > That's good point. We can do more benchmarks when more fixes are
    > > available. However I suspect their design goal will be "fix them
    > > without introducing noticeable overheads" :)
    >
    > s/noticeable//
    >
    > The problem with all the non-noticeable overheads that we're
    > continually adding to the kernel is that we're adding them to
    > the kernel. Non-noticeable part only makes it worse because
    > you can't bisect them :)

    Yes it makes sense.

    > > > One other thing to keep in mind that I will mention is that I am
    > > > going to push in a patch to the page allocator to allow callers
    > > > to avoid the refcounting (atomic_dec_and_test) in page lifetime,
    > > > which is especially important for SLUB and takes more cycles off
    > > > the page allocator...
    > > >
    > > > I don't know exactly what you're going to do after that to get a
    > > > stable reference to slab pages. I guess you can read the page
    > > > flags and speculatively take some slab locks and recheck etc...
    > >
    > > For reliably we could skip page lock on zero refcounted pages.
    > >
    > > We may lose the PG_hwpoison bit on races with __SetPageSlub*, however
    > > it should be an acceptable imperfection.
    >
    > I think if you're wiling to accept these problems, then it is
    > completely reasonable to also accept similar races with kernel
    > fastpaths to avoid extra overheads there.

    Yes I do. Even better, for this perticular race, we managed to avoid
    it completely without introducing overheads in fast path :)

    Thanks,
    Fengguang



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