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    SubjectRe: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
    On Fri, 2009-07-10 at 21:45 +0900, wrote:
    > From: Andi Kleen <>
    > Subject: Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
    > Date: Mon, 6 Jul 2009 13:54:51 +0200
    > Thank you for your replying, Peter and Andi.
    > > > Maybe re-use the LOCK_CONTENDED macros for this, but I'm not sure we
    > > > want to go there and put code like this on the lock hot-paths for !debug
    > > > kernels.
    > >
    > > My concern was similar.
    > >
    > > I suspect it would be in theory ok for the slow spinning path, but I am
    > > somewhat concerned about the additional cache miss for checking
    > > the global flag even in this case. This could hurt when
    > > the kernel is running fully cache hold, in that the cache miss
    > > might be far more expensive that short spin.
    > Yes, there will be overhead. This is certain.
    > But there's the radical way to ignore this,
    > adding subcategory to Kconfig for measuring spinlocks and #ifdef to spinlock.c.
    > So people who wants to avoid this overhead can disable measurement of spinlocks completely.
    > And there's another way to avoid the overhead of measurement.
    > Making _spin_lock variable of function pointer.
    > When you don't want to measure spinlocks,
    > assign _spin_lock_raw() which is equals to current _spin_lock().
    > When you want to measure spinlocks,
    > assign _spin_lock_perf() which locks and measures.
    > This way will banish the cache miss problem you said.
    > I think this may be useful for avoiding problem of recursion.

    We already have that, its called CONFIG_LOCKDEP && CONFIG_EVENT_TRACING
    && CONFIG_EVENT_PROFILE, with those enabled you get tracepoints on every
    lock acquire and lock release, and perf can already use those as event

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