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    SubjectRe: [GIT PULL] adaptive spinning mutexes

    * Andrew Morton <akpm@linux-foundation.org> wrote:

    > On Wed, 14 Jan 2009 21:51:22 +0100
    > Ingo Molnar <mingo@elte.hu> wrote:
    >
    > >
    > > * Andrew Morton <akpm@linux-foundation.org> wrote:
    > >
    > > > > > Do people enable CONFIG_SCHED_DEBUG?
    > > > >
    > > > > If they suspect performance problems and want to analyze them?
    > > >
    > > > The vast majority of users do not and usually cannot compile their own
    > > > kernels.
    > >
    > > ... which they derive from distro kernels or some old .config they always
    > > used, via 'make oldconfig'. You are arguing against well-established facts
    > > here.
    > >
    > > If you dont believe my word for it, here's an analysis of all kernel
    > > configs posted to lkml in the past 8 months:
    > >
    > > $ grep ^CONFIG_SCHED_DEBUG linux-kernel | wc -l
    > > 424
    > >
    > > $ grep 'CONFIG_SCHED_DEBUG is not' linux-kernel | wc -l
    > > 109
    > >
    > > i.e. CONFIG_SCHED_DEBUG=y is set in 80% of the configs. A large majority
    > > of testers has it enabled and /sys/debug/sched_features was always a good
    > > mechanism that we used for runtime toggles.
    >
    > You just disproved your own case :(

    how so? 80% is not enough? I also checked Fedora and it has SCHED_DEBUG=y
    in its kernel rpms.

    note that there's also a performance issue here: we generally _dont want_
    a debug sysctl overhead in the mutex code or in any fastpath for that
    matter. So making it depend on SCHED_DEBUG is useful.

    sched_feat() features get optimized out at build time when SCHED_DEBUG is
    disabled. So it gives us the best of two worlds: the utility of sysctls in
    the SCHED_DEBUG=y, and they get compiled out in the !SCHED_DEBUG case.

    Ingo


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