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Date: Wed, 5 Jul 2006 08:35:50 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Williams <pwil3058@bigpond.net.au>
Cc: Andrew Morton <akpm@osdl.org>, Nick Piggin <nickpiggin@yahoo.com.au>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Con Kolivas <kernel@kolivas.org>
Subject: Re: [PATCH] sched: Add SCHED_BGND (background) scheduling policy
Message-ID: <20060705063550.GA28004@elte.hu>
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* Peter Williams <pwil3058@bigpond.net.au> wrote:

> ===================================================================
> --- MM-2.6.17-mm6.orig/kernel/mutex.c	2006-07-04 14:37:43.000000000 +1000
> +++ MM-2.6.17-mm6/kernel/mutex.c	2006-07-04 14:38:12.000000000 +1000
> @@ -51,6 +51,16 @@ __mutex_init(struct mutex *lock, const c
>  
>  EXPORT_SYMBOL(__mutex_init);
>  
> +static inline void inc_mutex_count(void)
> +{
> +	current->mutexes_held++;
> +}
> +
> +static inline void dec_mutex_count(void)
> +{
> +	current->mutexes_held--;
> +}
> +

NACK! This whole patch is way too intrusive for such a relatively small 
gain.

also, if something doesnt hold a mutex, it might still be unsafe to 
background it! For example if it holds a semaphore. Or an rwsem. Or any 
other kernel resource that has exclusion semantics.

so unless this patch gets _much_ less complex and much less intrusive, 
we'll have to stay with SCHED_BATCH and nice +19.

	Ingo