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    SubjectRe: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
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    From: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Subject: Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
    Date: Fri, 10 Jul 2009 14:52:16 +0200

    > 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.
    >
    >
    >
    >

    Thanks, I understood your advice. Using infrastructure of ftrace is good idea, so I'll use it.

    But I have a question.
    I can't enable CONFIG_EVENT_PROFILE because it depends on CONFIG_EVENT_TRACER.
    And CONFIG_EVENT_TRACER seems something never enabled.

    % git grep EVENT_TRACER
    arch/arm/configs/cm_x300_defconfig:# CONFIG_EVENT_TRACER is not set
    arch/arm/configs/davinci_all_defconfig:# CONFIG_EVENT_TRACER is not set
    arch/arm/configs/ep93xx_defconfig:# CONFIG_EVENT_TRACER is not set
    ...
    arch/x86/configs/i386_defconfig:# CONFIG_EVENT_TRACER is not set
    arch/x86/configs/x86_64_defconfig:# CONFIG_EVENT_TRACER is not set
    init/Kconfig: depends on PERF_COUNTERS && EVENT_TRACER

    In addition, this is the output of searching this on menuconfig
    Symbol: EVENT_TRACER [=EVENT_TRACER]
    and, there is a log in git

    commit a7abe97fd8e7a6ccabba5a04a9f17be9211d418c
    Author: Steven Rostedt <srostedt@redhat.com>
    Date: Mon Apr 20 10:59:34 2009 -0400

    tracing: rename EVENT_TRACER config to ENABLE_EVENT_TRACING

    Currently we have two configs: EVENT_TRACING and EVENT_TRACER.
    All tracers enable EVENT_TRACING. The EVENT_TRACER is only a
    convenience to enable the EVENT_TRACING when no other tracers
    are enabled.

    Does EVENT_TRACER make any sense?
    If doesn't, can I remove dependency of CONFIG_EVENT_PROFILE?


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