Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth

Peter Zijlstra
2019-11-01T13:14:37Z

On Fri, Nov 01, 2019 at 05:58:16AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:08:06AM +0000, Lai Jiangshan wrote:
> > +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */
> > +static __always_inline int rcu_preempt_depth(void)
> > +{
> > + return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL;
>
> Why not raw_cpu_generic_read()?
>
> OK, OK, I get that raw_cpu_read_4() translates directly into an "mov"
> instruction on x86, but given that x86 percpu_from_op() is able to
> adjust based on operand size, why doesn't something like raw_cpu_read()
> also have an x86-specific definition that adjusts based on operand size?

The reason for preempt.h was header recursion hell.

> > +}
> > +
> > +static __always_inline void rcu_preempt_depth_set(int pc)
> > +{
> > + int old, new;
> > +
> > + do {
> > + old = raw_cpu_read_4(__rcu_preempt_depth);
> > + new = (old & RCU_NEED_SPECIAL) |
> > + (pc & ~RCU_NEED_SPECIAL);
> > + } while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old);
>
> Ummm...
>
> OK, as you know, I have long wanted _rcu_read_lock() to be inlineable.
> But are you -sure- that an x86 cmpxchg is faster than a function call
> and return? I have strong doubts on that score.

This is a regular CMPXCHG instruction, not a LOCK prefixed one, and that
should make all the difference

> Plus multiplying the x86-specific code by 26 doesn't look good.
>
> And the RCU read-side nesting depth really is a per-task thing. Copying
> it to and from the task at context-switch time might make sense if we
> had a serious optimization, but it does not appear that we do.
>
> You original patch some years back, ill-received though it was at the
> time, is looking rather good by comparison. Plus it did not require
> architecture-specific code!

Right, so the per-cpu preempt_count code relies on the preempt_count
being invariant over context switches. That means we never have to
save/restore the thing.

For (preemptible) rcu, this is 'obviously' not the case.

That said, I've not looked over this patch series, I only got 1 actual
patch, not the whole series, and I've not had time to go dig out the
rest..