On Wed, 21 Nov 2018, Andi Kleen wrote: > > + * This could be optimized by keeping track of the last > > + * user task per cpu and avoiding the barrier when the task > > + * is immediately scheduled back and the thread inbetween > > + * was a kernel thread. It's dubious whether that'd be > > + * worth the extra load/store and conditional operations. > > + * Keep it optimized for the common case where the TIF bit > > + * is not set. > > + */ > > The optimization was there before and you removed it? > > It's quite important for switching to idle and back. With your variant short > IOs > that do short idle waits will be badly impacted.
The question is what scenario to optimize for. Either you penalize everybody in the default prctl+seccomp setup (irrespective of it's TIF flag value), as you have the extra overhead on each and every switch_to() (to check exactly for this back-to-back scheduling), or you penalize only those tasks that are penalized anyway by the IBPB flush. I think the latter (which is what this patch implements) makes more sense. Thanks, -- Jiri Kosina SUSE Labs
