Re: [PATCH -v3 5/5] oom, PM: make OOM detection in the freezer path raceless

[Tejun Heo]

On Fri, Jan 09, 2015 at 12:05:55PM +0100, Michal Hocko wrote:
...
> @@ -142,7 +118,6 @@ static bool check_frozen_processes(void)
>  int freeze_processes(void)
>  {
>  	int error;
> -	int oom_kills_saved;
>  
>  	error = __usermodehelper_disable(UMH_FREEZING);
>  	if (error)
> @@ -157,29 +132,22 @@ int freeze_processes(void)
>  	pm_wakeup_clear();
>  	pr_info("Freezing user space processes ... ");
>  	pm_freezing = true;
> -	oom_kills_saved = oom_kills_count();
>  	error = try_to_freeze_tasks(true);
>  	if (!error) {
>  		__usermodehelper_set_disable_depth(UMH_DISABLED);
> -		oom_killer_disable();
> -
> -		/*
> -		 * There might have been an OOM kill while we were
> -		 * freezing tasks and the killed task might be still
> -		 * on the way out so we have to double check for race.
> -		 */
> -		if (oom_kills_count() != oom_kills_saved &&
> -		    !check_frozen_processes()) {
> -			__usermodehelper_set_disable_depth(UMH_ENABLED);
> -			pr_cont("OOM in progress.");
> -			error = -EBUSY;
> -		} else {
> -			pr_cont("done.");
> -		}
> +		pr_cont("done.");
>  	}
>  	pr_cont("\n");
>  	BUG_ON(in_atomic());
>  
> +	/*
> +	 * Now that the whole userspace is frozen we need to disbale
> +	 * the OOM killer to disallow any further interference with
> +	 * killable tasks.
> +	 */
> +	if (!error && !oom_killer_disable())

So, previously, oom killer was disabled at the top of
freeze_kernel_threads(), right?  I think that was the better spot to
do that.  We don't want to disable oom killer before the system is
just about to enter total quiescence which is freeze_kernel_threads().
We want to delay this as long as possible.  Let's please disable oom
killing in at the top of freeze_kernel_threads() and re-enable at the
bottom of thaw_kernel_threads().

Thanks.

-- 
tejun
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