Re: [PATCH 23/28] rcuscale: Add hazptr scale type

[KunWu Chan]

On Wed, Nov 11, 2026 at 5:45 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Nov 05, 2026 at 12:54:41AM +0800, Kunwu Chan wrote:
> > Add a hazptr scale type for per-writer hazptr_synchronize()
> > cost and per-reader hazptr_acquire()/hazptr_release() overhead.
> >
> > Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > ---
> >  kernel/rcu/rcuscale.c | 67 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 67 insertions(+)
> >
> > diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
> > index 5659eaebc5c0..6e3e3f296f85 100644
> > --- a/kernel/rcu/rcuscale.c
> > +++ b/kernel/rcu/rcuscale.c
> > @@ -40,6 +40,7 @@
> >  #include <linux/vmalloc.h>
> >  #include <linux/rcupdate_trace.h>
> >  #include <linux/sched/debug.h>
> > +#include <linux/hazptr.h>
> >
> >  #include "rcu.h"
> >
> > @@ -460,6 +461,71 @@ static struct rcu_scale_ops tasks_tracing_ops = {
> >
> >  #endif // #else // #ifdef CONFIG_TASKS_TRACE_RCU
> >
> > +#if IS_ENABLED(CONFIG_HAZPTR_TORTURE_TEST)
>
> Ah, maybe this is why you want hazptrtorture and rcuscale to be built
> into the same kernel.
>
> But what you really want is hazard pointers to be built in, right?
> As opposed to the torture test?  But this patch will be on top of the
> patches providing unconditional hazard-pointer support in the kernel,
> so you can just omit the above #ifdef.
>
> The reason for the CONFIG_HAZPTR_TORTURE_TEST #ifdef is that it is
> possible to build kernels that do not support Tasks Trace RCU.
>

Hi Paul,

Yes, that guard is what led me to enable hazptrtorture for the
rcuscale runs in patch 26. rcuscale only needs the hazptr core,
which is available unconditionally at this point in the series.

I'll remove the guard and add &hazptr_scale_ops directly to
scale_ops[]. This also removes the need for the torture_type=none
workaround.

Thanks,
Kunwu

>                                                         Thanx, Paul
>
> > +#define HAZPTR_SCALE_NR_OBJS 8
> > +static int hazptr_scale_obj[HAZPTR_SCALE_NR_OBJS];
> > +static atomic_t hazptr_scale_sync_idx = ATOMIC_INIT(0);
> > +
> > +struct hazptr_scale_state {
> > +     struct hazptr_ctx ctx;
> > +     void *addr;
> > +};
> > +static DEFINE_PER_CPU(struct hazptr_scale_state, hazptr_scale_state);
> > +
> > +static int hazptr_scale_read_lock(void)
> > +{
> > +     struct hazptr_scale_state *state = this_cpu_ptr(&hazptr_scale_state);
> > +     int idx = smp_processor_id() % HAZPTR_SCALE_NR_OBJS;
> > +     void *ptr = &hazptr_scale_obj[idx];
> > +
> > +     state->addr = hazptr_acquire(&state->ctx, &ptr);
> > +     return 0;
> > +}
> > +
> > +static void hazptr_scale_read_unlock(int idx)
> > +{
> > +     struct hazptr_scale_state *state = this_cpu_ptr(&hazptr_scale_state);
> > +
> > +     hazptr_release(&state->ctx, state->addr);
> > +}
> > +
> > +static unsigned long hazptr_scale_completed(void)
> > +{
> > +     return 0;
> > +}
> > +
> > +static void hazptr_scale_sync(void)
> > +{
> > +     int idx = atomic_fetch_inc(&hazptr_scale_sync_idx) % HAZPTR_SCALE_NR_OBJS;
> > +
> > +     hazptr_synchronize(&hazptr_scale_obj[idx]);
> > +}
> > +
> > +static void hazptr_scale_sync_exp(void)
> > +{
> > +     int idx = atomic_fetch_inc(&hazptr_scale_sync_idx) % HAZPTR_SCALE_NR_OBJS;
> > +
> > +     hazptr_synchronize(&hazptr_scale_obj[idx]);
> > +}
> > +
> > +static struct rcu_scale_ops hazptr_scale_ops = {
> > +     .ptype          = 0,
> > +     .readlock       = hazptr_scale_read_lock,
> > +     .readunlock     = hazptr_scale_read_unlock,
> > +     .get_gp_seq     = hazptr_scale_completed,
> > +     .gp_diff        = NULL,
> > +     .exp_completed  = hazptr_scale_completed,
> > +     .sync           = hazptr_scale_sync,
> > +     .exp_sync       = hazptr_scale_sync_exp,
> > +     .name           = "hazptr",
> > +};
> > +
> > +#define HAZPTR_SCALE_OPS &hazptr_scale_ops,
> > +#else
> > +#define HAZPTR_SCALE_OPS
> > +#endif
> > +
> >  static unsigned long rcuscale_seq_diff(unsigned long new, unsigned long old)
> >  {
> >       if (!cur_ops->gp_diff)
> > @@ -1140,6 +1206,7 @@ rcu_scale_init(void)
> >       static struct rcu_scale_ops *scale_ops[] = {
> >               &rcu_ops, &srcu_ops, &srcua_ops, &srcud_ops,
> >               TASKS_OPS TASKS_RUDE_OPS TASKS_TRACING_OPS
> > +             HAZPTR_SCALE_OPS
> >       };
> >
> >       if (!torture_init_begin(scale_type, verbose))
> > --
> > 2.43.0
> >