Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline

[David Laight <david.laight.linux@xxxxxxxxx>]

On Sat, 12 Sep 2026 15:31:24 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> On Sat, Sep 12, 2026 at 10:14:00PM +0100, David Laight wrote:
> > On Sat, 12 Sep 2026 11:03:34 -0700
> > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> >   
> > > In the old kernels, yes, we have current->trc_reader_nesting++.
> > > In the newer kernels, Tasks Trace RCU is instead implemented in terms
> > > of SRCU-fast, which instead increments per-CPU counters.  Which among
> > > other thins is a bit faster and does not need to hook into the scheduler. 
> > >  
> > 
> > Isn't that rather architecture dependant?
> > It is fine on x86, but on arm incrementing a per-cpu variable is
> > significantly expensive.  
> 
> Last I heard, slow ARM increments of per-CPU variables were to be a
> transitory phenomemon.  Plus changes late last year greatly sped up the
> per-CPU increment operations.

There are some unapplied patches to improve per-cpu operations for both
arm64 and s390.
Without those preemption has to be disabled (in current->xxx) which requires
a conditional call in the preempt enable path.

David

> Plus this change removed some hundreds
> of lines of RCU code.
> 
>                                                       Thanx, Paul
>