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

Paul E. McKenney
2019-11-01T14:30:40Z

On Fri, Nov 01, 2019 at 02:13:15PM +0100, Peter Zijlstra wrote:
> 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.

Fair enough, being as that is also the reason for _rcu_read_lock()
not being inlined. :-/

> > > +}
> > > +
> > > +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

Yes, understood, but this is also adding some arithmetic, a comparison,
and a conditional branch. Are you -sure- that this is cheaper than
an unconditional call and return?

> > 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..

I have taken a couple of the earlier patches in the series.

Perhaps inlining these things is instead a job for the long anticipated
GCC LTO? ;-)

Thanx, Paul