Re: [PATCH 1/2] do CPU_DEAD migrating under read_lock(tasklist)instead of write_lock_irq(tasklist)

[Andrew Morton]

On Fri, 24 Aug 2007 20:53:03 +0400
Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

> (the explicit ack/nack from maintainers is wanted)

It's not completely clear who "maintainers" refers to when it comes to this
code.

> Currently move_task_off_dead_cpu() is called under write_lock_irq(tasklist).
> This means it can't use task_lock() which is needed to improve migrating to
> take task's ->cpuset into account.
> 
> Change the code to call move_task_off_dead_cpu() with irqs enabled, and change
> migrate_live_tasks() to use read_lock(tasklist).
> 
> This all is a preparation for the futher changes proposed by Cliff Wickman, see
> 	http://marc.info/?t=117327786100003
> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> 
> --- t/kernel/sched.c~1_READ_LOCK	2007-08-12 14:15:37.000000000 +0400
> +++ t/kernel/sched.c	2007-08-24 20:31:03.000000000 +0400
> @@ -5048,7 +5048,7 @@ static void move_task_off_dead_cpu(int d
>  	unsigned long flags;
>  	cpumask_t mask;
>  	struct rq *rq;
> -	int dest_cpu;
> +	int dest_cpu, done;
>  
>  restart:
>  	/* On same node? */
> @@ -5077,7 +5077,11 @@ restart:
>  			       "longer affine to cpu%d\n",
>  			       p->pid, p->comm, dead_cpu);
>  	}
> -	if (!__migrate_task(p, dead_cpu, dest_cpu))
> +
> +	local_irq_disable();
> +	done = __migrate_task(p, dead_cpu, dest_cpu);
> +	local_irq_enable();
> +	if (!done)
>  		goto restart;
>  }
>  
> @@ -5106,7 +5110,7 @@ static void migrate_live_tasks(int src_c
>  {
>  	struct task_struct *p, *t;
>  
> -	write_lock_irq(&tasklist_lock);
> +	read_lock(&tasklist_lock);
>  
>  	do_each_thread(t, p) {
>  		if (p == current)
> @@ -5116,7 +5120,7 @@ static void migrate_live_tasks(int src_c
>  			move_task_off_dead_cpu(src_cpu, p);
>  	} while_each_thread(t, p);
>  
> -	write_unlock_irq(&tasklist_lock);
> +	read_unlock(&tasklist_lock);
>  }
>  
>  /*
> @@ -5180,11 +5184,10 @@ static void migrate_dead(unsigned int de
>  	 * Drop lock around migration; if someone else moves it,
>  	 * that's OK.  No task can be added to this CPU, so iteration is
>  	 * fine.
> -	 * NOTE: interrupts should be left disabled  --dev@
>  	 */
> -	spin_unlock(&rq->lock);
> +	spin_unlock_irq(&rq->lock);
>  	move_task_off_dead_cpu(dead_cpu, p);
> -	spin_lock(&rq->lock);
> +	spin_lock_irq(&rq->lock);
>  
>  	put_task_struct(p);
>  }

fyi, with a little rework I queued these changes behind Akinobu Mita's
cpu-hotplug changes:

cpu-hotplug-slab-cleanup-cpuup_callback.patch
cpu-hotplug-slab-fix-memory-leak-in-cpu-hotplug-error-path.patch
cpu-hotplug-cpu-deliver-cpu_up_canceled-only-to-notify_oked-callbacks-with-cpu_up_prepare.patch
cpu-hotplug-topology-remove-topology_dev_map.patch
cpu-hotplug-thermal_throttle-fix-cpu-hotplug-error-handling.patch
cpu-hotplug-msr-fix-cpu-hotplug-error-handling.patch
cpu-hotplug-cpuid-fix-cpu-hotplug-error-handling.patch
cpu-hotplug-mce-fix-cpu-hotplug-error-handling.patch
cpu-hotplug-intel_cacheinfo-fix-cpu-hotplug-error-handling.patch
cpu-hotplug-intel_cacheinfo-fix-cpu-hotplug-error-handling-fix-a-section-mismatc

Lots of other patches keep on coming in and trashing his changes and it's
getting a bit bothersome.
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