On Tue, Sep 03, 2024 at 05:38:05PM -0400, Kent Overstreet wrote: > 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? Yes, there are no smp_mb() invocations in either srcu_read_lock_lite() or srcu_read_unlock_lite(). As usual, nothing comes for free, so the overhead is moved to the update side, and amplified, in the guise of the at least two calls to synchronize_rcu(). > > 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. Glad you like it! > 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. Thank you, but I will pass on more accounts. I have a fair amount of hardware at my disposal. ;-) Thanx, Paul
