Re: [PATCH] kernel/smp: Improve smp_call_function_single() CSD-lock diagnostics

[Paul E. McKenney]

On Wed, Mar 25, 2026 at 12:32:16PM +0100, Ulf Hansson wrote:
> On Fri, 20 Mar 2026 at 11:45, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > Both smp_call_function() and smp_call_function_single() use per-CPU
> > call_single_data_t variable to hold the infamous CSD lock.  However,
> > while smp_call_function() acquires the destination CPU's CSD lock,
> > smp_call_function_single() instead uses the source CPU's CSD lock.
> > (These are two separate sets of CSD locks, cfd_data and csd_data,
> > respectively.)
> >
> > This otherwise inexplicable pair of choices is explained by their
> > respective queueing properties.  If smp_call_function() where to
> > use the sending CPU's CSD lock, that would serialize the destination
> > CPUs' IPI handlers and result in long smp_call_function() latencies,
> > especially on systems with large numbers of CPUs.  For its part, if
> > smp_call_function_single() were to use the (single) destination CPU's
> > CSD lock, this would similarly serialize in the case where many CPUs
> > are sending IPIs to a single "victim" CPU.  Plus it would result in
> > higher levels of memory contention.
> >
> > Except that if you don't have NMI-based stack tracing and you are working
> > with a weakly ordered system where remote unsynchronized stack traces are
> > especially unreliable, the improved debugging beats the improved queueing.
> > Keep in mind that this improved queueing only matters if a bunch of
> > CPUs are calling smp_call_function_single() concurrently for a single
> > "victim" CPU, which is not the common case.
> >
> > Therefore, make smp_call_function_single() use the destination CPU's
> > csd_data instance in kernels built with CONFIG_CSD_LOCK_WAIT_DEBUG=y
> > where csdlock_debug_enabled is also set.  Otherwise, continue to use
> > the source CPU's csd_data.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> FWIW, feel free to add:
> 
> Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>

Thank you!  I will apply this on my next rebase.

							Thanx, Paul

> Kind regards
> Uffe
> 
> >
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index f349960f79cad..09521cb38a55c 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -377,6 +377,20 @@ static __always_inline void csd_unlock(call_single_data_t *csd)
> >
> >  static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
> >
> > +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
> > +static call_single_data_t *get_single_csd_data(int cpu)
> > +{
> > +       if (static_branch_unlikely(&csdlock_debug_enabled))
> > +               return per_cpu_ptr(&csd_data, cpu);
> > +       return this_cpu_ptr(&csd_data);
> > +}
> > +#else
> > +static call_single_data_t *get_single_csd_data(int cpu)
> > +{
> > +       return this_cpu_ptr(&csd_data);
> > +}
> > +#endif
> > +
> >  void __smp_call_single_queue(int cpu, struct llist_node *node)
> >  {
> >         /*
> > @@ -670,7 +684,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >
> >         csd = &csd_stack;
> >         if (!wait) {
> > -               csd = this_cpu_ptr(&csd_data);
> > +               csd = get_single_csd_data(cpu);
> >                 csd_lock(csd);
> >         }
> >