linux-kernel - Re: [PATCH v2] softlockup: remove hung_task_check

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Message-ID: <20090122083457.GC7438@elte.hu>
Date:	Thu, 22 Jan 2009 09:34:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mandeep Singh Baines <msb@...gle.com>
Cc:	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	rientjes@...gle.com, mbligh@...gle.com, thockin@...gle.com
Subject: Re: [PATCH v2] softlockup: remove hung_task_check_count


* Mandeep Singh Baines <msb@...gle.com> wrote:

> > do the need_resched() check first (it's very lighweight) - and thus 
> > only do the heavy ops (get-task-struct & tasklist_lock unlock) if that 
> > is set?
> 
> Wanted to upper-bound the amount of time the lock is held. In order to 
> give others a chance to write_lock the tasklist, released the lock 
> regardless of whether a re-schedule is need.

but this:

> +static void check_hung_reschedule(struct task_struct *t)
> +{
> +	get_task_struct(t);
> +	read_unlock(&tasklist_lock);
> +	if (need_resched())
> +		schedule();
> +	read_lock(&tasklist_lock);
> +	put_task_struct(t);
> +}

does not actually achieve that. Releasing a lock does not mean that other 
CPUs will immediately be able to get it - if the ex-owner quickly 
re-acquires it then it will often succeed in doing so. Perhaps adding a 
cpu_relax() would increase the chance ... but still, it looks a bit weird.

	Ingo
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