linux-kernel - Re: [patch] timer: Fix timers_update_migration(), and call it in tmigr

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Message-Id: <20170429180638.GI3956@linux.vnet.ibm.com>
Date:   Sat, 29 Apr 2017 11:06:38 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mike Galbraith <efault@....de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...e.hu>,
        Thomas Gleixner <tglx@...utronix.de>,
        PeterZijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [patch] timer: Fix timers_update_migration(), and call it in
 tmigr_init()

On Sat, Apr 29, 2017 at 06:06:40PM +0200, Mike Galbraith wrote:
> Note: there is more.  With this applied, my desktop box will no longer
> reproduce when booted to init 3 with nowatchdog on the command line. 
>  My 8 socket DL980 OTOH still will, though it takes longer, and is
> seemingly no longer interested in following up  with a permanent RCU
> stall after the tsc clocksource is killed, as it does in virgin source.
> 
> ---
> 
> timers_update_migration() is called by tick_nohz_activate() before
> the late initcall tmigr_init() sets tmigr_enabled to true, resulting
> in it updating neither timer_base.nohz_active nor .migration_enabled,
> meaning we'll not kick an idling cpu in add_timer_on().
> 
> Remove redundant loop avoidance such that tick_nohz_activate() updates
> timer_bases[].nohz_active as intended, and call it in tmigr_init() to
> update timer_bases[].migration_enabled.
> 
> Signed-off-by: Mike Galbraith <efault@....de>
> Fixes: ec2206b91d43 timer: Implement the hierarchical pull model

If someone will either repost a fresh series or point me at exactly
the set of patches to use, I will run it through rcutorture again.

							Thanx, Paul

> ---
>  kernel/time/timer.c           |    4 ----
>  kernel/time/timer_migration.c |    1 +
>  2 files changed, 1 insertion(+), 4 deletions(-)
> 
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -224,10 +224,6 @@ void timers_update_migration(bool update
>  	bool on = sysctl_timer_migration && tick_nohz_active && tmigr_enabled;
>  	unsigned int cpu;
> 
> -	/* Avoid the loop, if nothing to update */
> -	if (this_cpu_read(timer_bases[BASE_GLOBAL].migration_enabled) == on)
> -		return;
> -
>  	for_each_possible_cpu(cpu) {
>  		per_cpu(timer_bases[BASE_LOCAL].migration_enabled, cpu) = on;
>  		per_cpu(timer_bases[BASE_GLOBAL].migration_enabled, cpu) = on;
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -649,6 +649,7 @@ static int __init tmigr_init(void)
>  		goto hp_err;
> 
>  	tmigr_enabled = true;
> +	timers_update_migration(false);
>  	pr_info("Timer migration: %d hierarchy levels\n", tmigr_hierarchy_levels);
>  	return 0;
> 
>