Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

[Peter Zijlstra]

On Thu, Jan 25, 2018 at 09:58:20AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 24, 2018 at 04:36:41PM -0800, Tim Chen wrote:
> > These two patches provide optimization to skip IBPB for this
> > commonly encountered scenario:
> > We could switch to a kernel idle thread and then back to the original
> > process such as:
> > process A -> idle -> process A
> > 
> > In such scenario, we do not have to do IBPB here even though the process
> > is non-dumpable, as we are switching back to the same process after
> > an hiatus.
> > 
> > The cost is to have an extra pointer to track the last mm we were using before
> > switching to the init_mm used by idle.  But avoiding the extra IBPB
> > is probably worth the extra memory for such a common scenario.
> 
> So we already track active_mm for kernel threads. I can't immediately
> see where this fails for idle and your changelog doesn't say.

idle_task_exit() explicitly switches back to init_mm when we take the
CPU offline, this very much suggests the active_mm thing works for idle
too.

This means that 'A -> idle -> A' should never pass through switch_mm to
begin with.

Please clarify how you think it does.