Re: [RFC] in-kernel rseq

[Heiko Carstens <hca@linux.ibm.com>]

On Fri, Apr 10, 2026 at 11:27:50PM +0530, Shrikanth Hegde wrote:
> Hi Peter,
> 
> On 2/23/26 10:08 PM, Peter Zijlstra wrote:
> > Hi,
> > 
> > It has come to my attention that various people are struggling with
> > preempt_disable()+preempt_enable() costs for various architectures.
> > 
> > Mostly in relation to things like this_cpu_ and or local_.
> > 
> > The below is a very crude (and broken, more on that below) POC.
> > 
> > So the 'main' advantage of this over preempt_disable()/preempt_enable(),
> > it on the preempt_enable() side, this elides the whole conditional and
> > call schedule() nonsense.
> 
> This might be a very stupid question.
> 
> Why not just call preempt_enable_no_resched instead?
> 
> this_cpu operations would have disabled preemption so the per_cpu
> structures are protected from thread migration right? intention
> here is likely not to call schedule. No?

Doing that would make it unpredictable when the kernel is preempted for
cases where TIF_PREEMPT is set while within a code section which has
preemption disabled and where preemption is enabled again without checking
for TIF_PREEMPT.