Messages in this thread | | | Date | Fri, 10 Jul 2009 15:50:32 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat |
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Fri, Jul 10, 2009 at 03:43:07PM +0200, Ingo Molnar wrote: > > > > * 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 > > > I wonder if the lock_*() events should become independant from > lockdep so that we don't need to always enable lockdep to get the > lock events at the same time. > > It could be a separate option.
They already should be to a large degree if CONFIG_LOCK_STAT is enabled but CONFIG_PROVE_LOCKING is off. In theory :-)
Ingo
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