Re: [PATCH v2 0/2] send tlb_remove_table_smp_sync IPI only to necessary CPUs

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jun 21, 2023 at 09:43:37AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 20, 2023 at 05:46:16PM +0300, Yair Podemsky wrote:
> > Currently the tlb_remove_table_smp_sync IPI is sent to all CPUs
> > indiscriminately, this causes unnecessary work and delays notable in
> > real-time use-cases and isolated cpus.
> > By limiting the IPI to only be sent to cpus referencing the effected
> > mm.
> > a config to differentiate architectures that support mm_cpumask from
> > those that don't will allow safe usage of this feature.
> > 
> > changes from -v1:
> > - Previous version included a patch to only send the IPI to CPU's with
> > context_tracking in the kernel space, this was removed due to race 
> > condition concerns.
> > - for archs that do not maintain mm_cpumask the mask used should be
> >  cpu_online_mask (Peter Zijlstra).
> >  
> 
> Would it not be much better to fix the root cause? As per the last time,
> there's patches that cure the thp abuse of this.

The other case where the IPI can happen is:

CPU-0                                   CPU-1

tlb_remove_table
tlb_remove_table_sync_one
IPI
                                        local_irq_disable
                                        gup_fast
                                        local_irq_enable


So its not only the THP case.