linux-kernel - Re: vmstat: On demand vmstat workers V8

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Date:	Tue, 29 Jul 2014 11:47:04 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Christoph Lameter <cl@...two.org>, Tejun Heo <tj@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org, hpa@...or.com, mingo@...nel.org,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: vmstat: On demand vmstat workers V8

On 07/29/2014 11:39 AM, Christoph Lameter wrote:
> On Tue, 29 Jul 2014, Tejun Heo wrote:
> 
>> Hmmm, well, then it's something else.  Either a bug in workqueue or in
>> the caller.  Given the track record, the latter is more likely.
>> e.g. it looks kinda suspicious that the work func is cleared after
>> cancel_delayed_work_sync() is called.  What happens if somebody tries
>> to schedule it inbetween?
> 
> Here is yet another patch to also address this idea:
> 
> Subject: vmstat: Clear the work.func before cancelling delayed work
> 
> Looks strange to me but Tejun thinks this could do some good.
> If this really is the right thing to do then cancel_delayed_work should
> zap the work func itselt I think.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> 
> Index: linux/mm/vmstat.c
> ===================================================================
> --- linux.orig/mm/vmstat.c	2014-07-29 10:22:45.073884943 -0500
> +++ linux/mm/vmstat.c	2014-07-29 10:34:45.083369228 -0500
> @@ -1277,8 +1277,8 @@ static int vmstat_cpuup_callback(struct
>  		break;
>  	case CPU_DOWN_PREPARE:
>  	case CPU_DOWN_PREPARE_FROZEN:
> -		cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
>  		per_cpu(vmstat_work, cpu).work.func = NULL;
> +		cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
>  		break;
>  	case CPU_DOWN_FAILED:
>  	case CPU_DOWN_FAILED_FROZEN:
> 

I'm slightly confused here. The on demand vmstat workers patch did this:

        case CPU_DOWN_PREPARE_FROZEN:
-               cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
-               per_cpu(vmstat_work, cpu).work.func = NULL;
+               if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
+                       cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));

So your new patch doesn't apply on top of it, and doesn't make sense before it.


Thanks,
Sasha
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