Re: [PATCH rcu 0/11] Add light-weight readers for SRCU

[Kent Overstreet]

On Tue, Sep 03, 2024 at 09:32:51AM GMT, Paul E. McKenney wrote:
> Hello!
> 
> This series provides light-weight readers for SRCU.  This lightness
> is selected by the caller by using the new srcu_read_lock_lite() and
> srcu_read_unlock_lite() flavors instead of the usual srcu_read_lock() and
> srcu_read_unlock() flavors.  Although this passes significant rcutorture
> testing, this should still be considered to be experimental.

This avoids memory barriers, correct?

> There are a few restrictions:  (1) If srcu_read_lock_lite() is called
> on a given srcu_struct structure, then no other flavor may be used on
> that srcu_struct structure, before, during, or after.  (2) The _lite()
> readers may only be invoked from regions of code where RCU is watching
> (as in those regions in which rcu_is_watching() returns true).  (3)
> There is no auto-expediting for srcu_struct structures that have
> been passed to _lite() readers.  (4) SRCU grace periods for _lite()
> srcu_struct structures invoke synchronize_rcu() at least twice, thus
> having longer latencies than their non-_lite() counterparts.  (5) Even
> with synchronize_srcu_expedited(), the resulting SRCU grace period
> will invoke synchronize_rcu() at least twice, as opposed to invoking
> the IPI-happy synchronize_rcu_expedited() function.  (6)  Just as with
> srcu_read_lock() and srcu_read_unlock(), the srcu_read_lock_lite() and
> srcu_read_unlock_lite() functions may not (repeat, *not*) be invoked
> from NMI handlers (that is what the _nmisafe() interface are for).
> Although one could imagine readers that were both _lite() and _nmisafe(),
> one might also imagine that the read-modify-write atomic operations that
> are needed by any NMI-safe SRCU read marker would make this unhelpful
> from a performance perspective.

So if I'm following, this should work fine for bcachefs, and be a nifty
small perforance boost.

Can I give you an account for my test cluster? If you'd like, we can
convert bcachefs to it and point it at your branch.