On Fri, Jul 20, 2018 at 01:05:59PM -0700, Davidlohr Bueso wrote: > On Fri, 20 Jul 2018, Andrew Morton wrote:
> >I'm surprised. Is spin_lock_irqsave() significantly more expensive > >than spin_lock_irq()? Relative to all the other stuff those functions > >are doing? If so, how come? Some architectural thing makes > >local_irq_save() much more costly than local_irq_disable()? > > For example, if you compare x86 native_restore_fl() to xen_restore_fl(), > the cost of Xen is much higher. Xen is a moot argument. IIRC the point is that POPF (as used by *irqrestore()) is a very expensive operation because it changes all flags and thus has very 'difficult' instruction dependencies, killing the front end reorder and generating a giant bubble in the pipeline. Similarly, I suppose PUSHF is an expensive instruction because it needs all the flags 'stable' and thus needs to wait for a fair number of prior instructions to retire before it can get on with it. Combined the whole PUSHF + POPF is _far_ more expensive than STI + CLI, because the latter only has dependencies on instructions that muck about with IF -- not that many.
