linux-kernel - Re: [linux-pm] [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space

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Date:	Wed, 26 May 2010 18:18:36 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Len Brown <len.brown@...el.com>,
	Jim Collar <jim.collar@...are.net>,
	<linux-doc@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...e.de>,
	<linux-kernel@...r.kernel.org>, Avi Kivity <avi@...hat.com>,
	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
	Magnus Damm <damm@...l.co.jp>,
	<linux-pm@...ts.linux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH 2/8] PM: suspend_block: Add driver to access
 suspend blockers from user-space

On Wed, 26 May 2010, Rafael J. Wysocki wrote:

> On Wednesday 26 May 2010, Peter Zijlstra wrote:
> > On Fri, 2010-05-21 at 15:46 -0700, Arve HjÃ¸nnevÃ¥g wrote:
> > > +To create a suspend blocker from user space, open the suspend_blocker
> > > special
> > > +device file:
> > > +
> > > +    fd = open("/dev/suspend_blocker", O_RDWR | O_CLOEXEC);
> > > +
> > > +then optionally call:
> > > +
> > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_SET_NAME(strlen(name)), name);
> > > +
> > > +To activate the suspend blocker call:
> > > +
> > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_BLOCK);
> > > +
> > > +To deactivate it call:
> > > +
> > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_UNBLOCK);
> > > +
> > > +To destroy the suspend blocker, close the device:
> > > +
> > > +    close(fd);
> > 
> > Urgh, please let the open() be BLOCK, the close() be UNBLOCK, and keep
> > the SET_NAME thing if you really care.
> 
> SET_NAME wouldn't serve any purpose in that case.
> 
> This whole thing is related to the statistics part, which Arve says is
> essential to him.  He wants to collect statistics for each suspend blocker
> activated and deactivated so that he can tell who's causing problems by
> blocking suspend too often.  The name also is a part of this.
> 
> In fact, without the statistics part the whole thing might be implemented
> as a single reference counter such that suspend would happen when it went down
> to zero.

There's also Arve's other main point: These suspend blockers tend to
get used quite a lot.  Opening and closing a file each time has much
higher overhead than a simple ioctl.

All these topics have been covered earlier in this discussion.  Peter 
should go back and read the emails in the linux-pm archive.

Alan Stern

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