linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Message-ID: <20131011182507.GA31625@redhat.com>
Date:	Fri, 11 Oct 2013 20:25:07 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On 10/11, Peter Zijlstra wrote:
>
> As a penance I'll start by removing all get_online_cpus() usage from the
> scheduler.

I only looked at the change in setaffinity,

> @@ -3706,7 +3707,6 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>  	struct task_struct *p;
>  	int retval;
>
> -	get_online_cpus();
>  	rcu_read_lock();

Hmm. In theory task_rq_lock() doesn't imply rcu-lock, so
set_cpus_allowed_ptr() can miss the change in cpu_active_mask. But this
is probably fine, CPU_DYING does __migrate_task().

However. This means that sched_setaffinity() can fail if it races with
the failing cpu_down() (say, __cpu_notify(CPU_DOWN_PREPARE) fails).
Probably we do not really care, just this looks a bit confusing.

> @@ -3814,7 +3813,6 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
>  	unsigned long flags;
>  	int retval;
>
> -	get_online_cpus();

This change is probably fine in any case?

>  	rcu_read_lock();
>
>  	retval = -ESRCH;
> @@ -3827,12 +3825,11 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
>  		goto out_unlock;
>
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	cpumask_and(mask, &p->cpus_allowed, cpu_online_mask);
> +	cpumask_and(mask, &p->cpus_allowed, cpu_active_mask);

But I am just curious, is this change is strictly needed?

Afaics we do not care if we race with set_cpu_online(true/false).

Oleg.

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