Re: [PATCH v3 0/9] s390: Improve this_cpu operations

[David Laight]

On Thu, 21 May 2026 09:57:37 -0700
Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> On 5/21/26 3:17 AM, David Laight wrote:
> > On Wed, 20 May 2026 17:23:37 -0700
> > Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >  
> >> On 5/20/26 3:34 PM, David Laight wrote:  
> > ...  
> >>> But I'm sure I remember that some cpu don't like having the same
> >>> physical address at different virtual addresses (and not just those
> >>> with VIVT caches like some sparc cpu).  
> >> Yeah, VIVT cache doesn't like it due to cache alias. But the mapping is
> >> really percpu, so the mapping to the physical address belonging to
> >> another CPU should never pollute the current CPU's cache if I don't miss
> >> something.
> >>  
> >>> I'm sure code can end up accessing the current cpu's percpu data
> >>> using the same address that other cpu use - there are definitely
> >>> places where it needs that address.  
> >> No, it is not. In the percpu page table approach, we use different
> >> address for this_cpu_*() and per_cpu_ptr() which is mainly used to
> >> initialize percpu data for all CPUs.  
> > You missed something.
> >
> > Look, for example, at kernel/locking/osq_lock.c
> > The code uses this_cpu_ptr() and then both dereferences the pointer
> > and writes it to places that other cpu will use.
> > It also uses per_cpu_ptr() to get an address it can use for the per-cpu
> > data of another cpu.
> > (That code all assumes preemption is disabled.)  
> 
> this_cpu_ptr() uses different addresses for different CPUs. It is a 
> special case, it is not due to VIVT, but because it may confuse list 
> API. Because list API determines list is empty by comparing pointers 
> (head->next == head). this_cpu_read/write/add/sub, etc, are fine.

But you could quite easily get code that manages to mix accesses through
this_cpu_ptr() with direct accesses to per-cpu variables in the same
cache line.
I'm sure some arm cpu really don't like you doing that.
(But it is a foggy memory from somewhere.)

You can use per-cpu page tables, but it really only helps for a
few items.
Anything that is RMW (eg add on pretty much everything except x86)
either has to disable preemption or use a compare and exchange loop.
Variables like 'current' can be written into the per-cpu page table
data area by the process switch code (as I believe s390 does).

The 'trick' here will work for reading/writing values if you don't
care that the value read is stale (or might have been written to
the memory for a different cpu).
It might work for updating the preemption disable count - because
you can only be preempted while it is zero.

-- David

> 
> And per_cpu_ptr() also uses different addresses for different CPUs.
> 
> The lwn article explained it. 
> https://lwn.net/SubscriberLink/1073395/12c08f128e515809/
> 
> Thanks,
> Yang
> 
> >
> > -- David
> >  
> >> Thanks,
> >> Yang  
>