linux-kernel - RE: Bug 71331 - mlock yields processor to lower priority process

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Date:	Fri, 21 Mar 2014 14:34:19 +0000
From:	<jimmie.davis@...com.com>
To:	<umgwanakikbuti@...il.com>
CC:	<artem_fetishev@...m.com>, <peterz@...radead.org>,
	<kosaki.motohiro@...fujitsu.com>, <linux-kernel@...r.kernel.org>
Subject: RE: Bug 71331 - mlock yields processor to lower priority process

________________________________________
From: Mike Galbraith [umgwanakikbuti@...il.com]
Sent: Friday, March 21, 2014 8:14 AM
To: Davis, Bud @ SSG - Link
Cc: artem_fetishev@...m.com; peterz@...radead.org; kosaki.motohiro@...fujitsu.com; linux-kernel@...r.kernel.org
Subject: RE: Bug 71331 - mlock yields processor to lower priority process

On Fri, 2014-03-21 at 12:18 +0000, jimmie.davis@...com.com wrote:

> As the submitter of the bug, let me give you my perspective.
> SCHED_FIFO means run my task until it blocks or a higher priority task
> pre-empts it.  Period.

It blocked.
>
> mlock() doesn't block. check the man page.
>
I don't see that specified.

(or how it could be, but what do I know, IANIPL)

> Any other way and you are not able to use priority based scheduling.

Sure you can, allocate and lock down resources before entering critical
sections.

If you think donning a SCHED_FIFO super-suit should make your task
unstoppable, you're gonna be very disappointed.  Fact is if your
Juggernaut bumps ever so gently into a contended sleeping variety lock
(and in the rt kernel that means nearly every lock), it will block.

-Mike

------------------------------------------------------------------------------------------------
Mike,

There are several problem domains where you protect critical sections by assigning multiple
threads to a single CPU and use priorities and SCHED_FIFO to ensure data integrity.

In this kind of design you don't make many syscalls.  The ones you do make, have to be clearly understood
if they block.   

So, yes, I expect that a SCHED_FIFO task, that uses a subset of syscalls known to be non-blocking, will not block.  

If it is not 'unstoppable', then there is a defect in the OS.

In the past, a call to mlock() was known to be OK.  It would not block.  It might take a while, but it would run to completion.  It does not do that any more.

If mlock() is now a blocking call, then fine.  It only needs to be called on occasion, and this can be accounted for
in the application design.  Does write() block ?  Yes, the man pages talks all about it.  Does clock_gettime() block ?
No, blocking is not mentioned in the man page.  Blocking behaviour is rare, when it exists it is documented.

My point is, this is either a defect to be fixed, or a change that warrants updating the documentation.

regards,
Bud Davis

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