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    SubjectRe: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat

    * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

    > On Fri, 2009-07-10 at 21:45 +0900, mitake@dcl.info.waseda.ac.jp wrote:
    > > From: Andi Kleen <andi@firstfloor.org>
    > > 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 sources.

    Yes, that could be reused for this facility too.

    Ingo


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