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Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline
- To: David Laight <david.laight.linux@xxxxxxxxx>
- From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
- Date: Sat, 12 Sep 2026 15:31:24 -0700
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- Delivery-date: Sat, 12 Sep 2026 22:31:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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. Plus this change removed some hundreds
of lines of RCU code.
Thanx, Paul
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