Date: Mon, 26 Jan 2009 14:32:07 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Frederic Weisbecker <fweisbec@gmail.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
       Mandeep Singh Baines <msb@google.com>
Subject: Re: [RFC][PATCH 2/2] add a counter for writers spinning on a rwlock
Message-ID: <20090126133207.GC13567@elte.hu>
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> This patch adds a counter for writers that enter a rwlock slow path. For 
> example it can be useful for slow background tasks which perform some 
> jobs on the tasklist, such as the hung_task detector 
> (kernel/hung_task.c).
> 
> It adds a inc/dec pair on the slow path and 4 bytes for each rwlocks, so 
> the overhead is not null.
> 
> Only x86 is supported for now, writers_spinning_lock() will return 0 on 
> other archs (which is perhaps not a good idea).
> 
> Comments?

hm, it increases the rwlock data type:

> diff --git a/arch/x86/include/asm/spinlock_types.h b/arch/x86/include/asm/spinlock_types.h
> index 845f81c..163e6de 100644
> --- a/arch/x86/include/asm/spinlock_types.h
> +++ b/arch/x86/include/asm/spinlock_types.h
> @@ -13,6 +13,7 @@ typedef struct raw_spinlock {
>  
>  typedef struct {
>  	unsigned int lock;
> +	unsigned int spinning_writers;
>  } raw_rwlock_t;

that's generally not done lightly. Performance figures for a relevant 
workload are obligatory in this case - proving that it's worth the size 
bloat.

	Ingo
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