Re: [linux-pm] [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: Memory allocations in .suspend became very unreliable)

Alan Stern
2010-01-19T15:14:04Z

On Mon, 18 Jan 2010, Rafael J. Wysocki wrote:

> On Monday 18 January 2010, Alan Stern wrote:
> > On Mon, 18 Jan 2010, Oliver Neukum wrote:
> >
> > > Am Montag, 18. Januar 2010 00:00:23 schrieb Rafael J. Wysocki:
> > > > On Sunday 17 January 2010, Benjamin Herrenschmidt wrote:
> > > > > On Sun, 2010-01-17 at 14:27 +0100, Rafael J. Wysocki wrote:
> > > > ...
> > > > > However, it's hard to deal with the case of allocations that have
> > > > > already started waiting for IOs. It might be possible to have some VM
> > > > > hook to make them wakeup, re-evaluate the situation and get out of that
> > > > > code path but in any case it would be tricky.
> > > >
> > > > In the second version of the patch I used an rwsem that made us wait for these
> > > > allocations to complete before we changed gfp_allowed_mask.
> > >
> > > This will be a very, very hot semaphore. What's the impact on performance?
> >
> > Can it be replaced with something having lower overhead, such as SRCU?
>
> I'm not sure about that. In principle SRCU shouldn't be used if the reader can
> sleep unpredictably long and the memory allocation sutiation is one of these.

I don't think this matters. Each SRCU usage has its own domain, and
different domains don't affect one another. So the only thing that
would be blocked by a long-sleeping reader would be the suspend process
itself, and obviously you won't mind that.

Alan Stern