On Thu, Apr 06, 2017 at 12:58:27PM +0200, Thomas Gleixner wrote: > On Thu, 6 Apr 2017, Peter Zijlstra wrote:
> > Because mainline will not get anything other than prempt_disable() for > > migrate_disable(). > > I'm not yet convinced of that. There is enough stuff which pointlessly > disables preemption and works around the restrictions caused by that or > plays games with cpus_allowed just to stay on a particular cpu. > > I know you don't like it because it imposes restrictions on schedulability, > but disabling preemption for a long time or playing cpus_allowed games has > the same and worse effects. There really isn't much code that plays games with cpus_allowed. And code that disabled preemption for a long time can surely be fixed, we've done so many a time. A pure migrate_disable() (without wrapping lock) is a straight up nightmare, you can end up with a gazillion runnable tasks stuck to one CPU and all other CPUs with their thumbs up their arses waiting for work. The moment you stick migrate_disable() in a lock, you at least get PI to bail you out of some of the pain (but by no means all of it). Adding migrate_disable() to the kernel is really just moving pain around, it doesn't solve anything much.