linux-kernel - Re: [v2 PATCH 4/4] timers: logic to enable timer migration.

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Date:	Thu, 5 Mar 2009 16:34:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	a.p.zijlstra@...llo.nl, ego@...ibm.com, tglx@...utronix.de,
	mingo@...e.hu, andi@...stfloor.org, venkatesh.pallipadi@...el.com,
	vatsa@...ux.vnet.ibm.com, arjan@...radead.org,
	svaidy@...ux.vnet.ibm.com
Subject: Re: [v2 PATCH 4/4] timers: logic to enable timer migration.

On 03/04, Arun R Bharadwaj wrote:
>
> @@ -628,7 +629,7 @@ __mod_timer(struct timer_list *timer, un
>  {
>  	struct tvec_base *base, *new_base;
>  	unsigned long flags;
> -	int ret;
> +	int ret, current_cpu, preferred_cpu;
>  
>  	ret = 0;
>  
> @@ -649,6 +650,16 @@ __mod_timer(struct timer_list *timer, un
>  
>  	new_base = __get_cpu_var(tvec_bases);
>  
> +	current_cpu = smp_processor_id();
> +	preferred_cpu = get_nohz_load_balancer();
> +	if (enable_timer_migration && !tbase_get_pinned(timer->base) &&
> +			idle_cpu(current_cpu) && preferred_cpu != -1) {
> +		new_base = per_cpu(tvec_bases, preferred_cpu);
> +		timer_set_base(timer, new_base);
> +		timer->expires = expires;
> +		internal_add_timer(new_base, timer);
> +		goto out_unlock;

I didn't read the whole series, but this looks very wrong.

We can not do internal_add_timer/etc until we lock new_base, please
look how the current code does this under "if (base != new_base)".

I think you can do something like

	-	new_base = __get_cpu_var(tvec_bases);
	+
	+	new_cpu = smp_processor_id();
	+	if (enable_timer_migration && ....)
	+		new_cpu = preferred_cpu;
	+
	+	new_base = per_cpu(tvec_bases, new_cpu);

		if (base != new_base) {

Oleg.

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