Re: [PATCH] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to MM CPUs

[ypodemsk]

On Mon, 2023-03-20 at 09:49 +0100, Peter Zijlstra wrote:
> On Sun, Mar 12, 2023 at 10:09:45AM +0200, 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, this patch will limit this
> > IPI to
> > only be sent to cpus referencing the effected mm and are currently
> > in
> > kernel space.
> 
> Did you validate that all architectures for which this is relevant
> actually set bits in mm_cpumask() ?
> 
Hi Peter,
Thank you for bringing this to my attention.
I reviewed the architectures using the MMU_GATHER_RCU_TABLE_FREE:
arm, powerpc, s390, sparc and x86 set the bit when switching process
in.
for arm64 removed set/clear bit in 38d96287504a ("arm64: mm: kill
mm_cpumask usage")
The reason given was that mm_cpumask was not used.
Given that we now have a use for it, I will add a patch to revert.
Thanks
Yair