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Date: Thu, 16 Sep 2004 21:30:13 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Zwane Mwaikambo <zwane@linuxpower.ca>
Cc: Andi Kleen <ak@suse.de>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, wli@holomorphy.com
Subject: Re: [PATCH] remove LOCK_SECTION from x86_64 spin_lock asm
Message-ID: <20040916193013.GA25730@elte.hu>
References: <Pine.LNX.4.53.0409151458470.10849@musoma.fsmlabs.com> <20040915144523.0fec2070.akpm@osdl.org> <20040916061359.GA12915@wotan.suse.de> <20040916062759.GA10527@elte.hu> <Pine.LNX.4.53.0409161238030.2897@musoma.fsmlabs.com>
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* Zwane Mwaikambo <zwane@linuxpower.ca> wrote:

> On Thu, 16 Sep 2004, Ingo Molnar wrote:
> 
> > the ebp trick is nice, but forcing a formal stack frame for every
> > function has global performance implications. Couldnt we define some
> > sort of current-> field [or current_thread_info() field] that the
> > spinlock code could set and clear, which field would be listened to by
> > profile_pc(), so that the time spent spinning would be attributed to the
> > callee? Something like:
> 
> I think the generic route is nice but wouldn't this break with the 
> following.
> 
> taskA:
> spin_lock(lockA); // contended
> <interrupt>
> int1:
> spin_lock(lockB)
> 
> I was thinking along the likes of a per_cpu real_pc, but realised it
> falls prey to the same problem as above... Unless we have irq threads,
> then of course your solution works.

you mean the nesting? spin_lock() should save/restore the value instead
of setting/clearing it - and fork() should initialize it to zero.

	Ingo
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