Re: [PATCH 0/4] Introduce QPW for per-cpu operations

[Marcelo Tosatti]

On Mon, Feb 23, 2026 at 10:11:11AM +0100, Michal Hocko wrote:
> On Fri 20-02-26 16:01:59, Marcelo Tosatti wrote:
> > On Fri, Feb 20, 2026 at 06:58:10PM +0100, Vlastimil Babka wrote:
> [...]
> > > >> So if we can assume that workloads on isolated cpus make syscalls only
> > > >> rarely, and when they do they can tolerate them being slower, I think the
> > > >> "avoid sheaves on isolated cpus" would be the best way here.
> > > > 
> > > > I am not sure its safe to assume that. Ask Gemini about isolcpus use
> > > > cases and:
> > > 
> > > I don't think it's answering the question about syscalls. But didn't read
> > > too closely given the nature of it.
> > 
> > People use isolcpus with all kinds of programs. 
> > 
> > > > For example, AF_XDP bypass uses system calls (and wants isolcpus):
> > > > 
> > > > https://www.quantvps.com/blog/kernel-bypass-in-hft?srsltid=AfmBOoryeSxuuZjzTJIC9O-Ag8x4gSwjs-V4Xukm2wQpGmwDJ6t4szuE
> > > 
> > > Didn't spot system calls mentioned TBH.
> > 
> > I don't see why you want to reduce performance of applications that 
> > execute on isolcpus=, if you can avoid that.
> 
> If you can avoid that by making performance bad for everybody else then
> then it seems safer to sacrifice those workloads that are much more
> special - i.e. cpu isolation.
> -- 
> Michal Hocko
> SUSE Labs

Performance is not bad for everyone else:

Without patchset:                                                                                                             
================                                                                                                              
                                                                                                                              
[ 1188.050725] kmalloc_bench: Avg cycles per kmalloc: 159                                                                     
                                                                                                                              
With qpw patchset, CONFIG_QPW=n:                                                                                              
================================                                                                                              
                                                                                                                              
[   50.292190] kmalloc_bench: Avg cycles per kmalloc: 163          

And its probably possible to remove those 4 cycles.

Which makes reduction of performance of isolcpus not necessary.