linux-kernel - Re: [PATCH 01/19] freezer: don't get over-anxious while waiting

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Date:	Fri, 02 Oct 2009 15:47:14 -0400
From:	Oren Laadan <orenl@...rato.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>,
	jeff@...zik.org, mingo@...e.hu, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [PATCH 01/19] freezer: don't get over-anxious while waiting



Rafael J. Wysocki wrote:
> On Thursday 01 October 2009, Pavel Machek wrote:
>>> Freezing isn't exactly the most latency sensitive operation and
>>> there's no reason to burn cpu cycles and power waiting for it to
>>> complete.  msleep(10) instead of yield().  This should improve
>>> reliability of emergency hibernation.
>> i don't see how it improves reliability, but its probably ok.
>>
>> Well... for hibernation anyway. I can imagine cgroup users where
>> freeze is so fast that this matters. rjw cc-ed.		pavel
> 
> Thanks.  I'd like to hear from the cgroup freezer people about that.
> 

[Adding Matt Helsley to the CC list]

To checkpoint or migrate an application, the cgroup to which it belongs
must be frozen first.

It's a bit down the road, but if one seeks minimum application downtime
during application checkpoint and/or migration, then a (minimum of)
10ms - or multiples of it - may result in a visible/undesired hick-up.

Perhaps avoid it when freezing a cgroup ?  or maybe a way for the user
to control this behavior per cgroup ?

Oren.

>>> Signed-off-by: Tejun Heo <tj@...nel.org>
>>> ---
>>>  kernel/power/process.c |   13 +++++++++----
>>>  1 files changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/kernel/power/process.c b/kernel/power/process.c
>>> index cc2e553..9d26a0a 100644
>>> --- a/kernel/power/process.c
>>> +++ b/kernel/power/process.c
>>> @@ -41,7 +41,7 @@ static int try_to_freeze_tasks(bool sig_only)
>>>  	do_gettimeofday(&start);
>>>  
>>>  	end_time = jiffies + TIMEOUT;
>>> -	do {
>>> +	while (true) {
>>>  		todo = 0;
>>>  		read_lock(&tasklist_lock);
>>>  		do_each_thread(g, p) {
>>> @@ -62,10 +62,15 @@ static int try_to_freeze_tasks(bool sig_only)
>>>  				todo++;
>>>  		} while_each_thread(g, p);
>>>  		read_unlock(&tasklist_lock);
>>> -		yield();			/* Yield is okay here */
>>> -		if (time_after(jiffies, end_time))
>>> +		if (!todo || time_after(jiffies, end_time))
>>>  			break;
>>> -	} while (todo);
>>> +
>>> +		/*
>>> +		 * We need to retry.  There's no reason to be
>>> +		 * over-anxious about it and waste power.
>>> +		 */
> 
> The comment above looks like it's only meaningful in the context of the patch.
> After it's been applied the meaning of the comment won't be so obvious, I'm
> afraid.
> 
>>> +		msleep(10);
>>> +	}
>>>  
>>>  	do_gettimeofday(&end);
>>>  	elapsed_csecs64 = timeval_to_ns(&end) - timeval_to_ns(&start);
> 
> Thanks,
> Rafael
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