Message-ID: <0ae7d150-7b92-4e8e-98ea-b28c3b824e11@paulmck-laptop>
Date: Wed, 4 Sep 2024 03:54:18 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, bpf@...r.kernel.org
Subject: Re: [PATCH rcu 0/11] Add light-weight readers for SRCU

On Tue, Sep 03, 2024 at 07:03:53PM -0400, Kent Overstreet wrote:
> On Tue, Sep 03, 2024 at 03:13:40PM GMT, Paul E. McKenney wrote:
> > 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.  ;-)
> 
> Well - bcachefs might be a good torture test; if you patch bcachefs to
> use the new API point me at a branch and I'll point the CI at it

I am sorry, but I am getting on a plane today, am short of time, and
won't be responsive for the inevitable issues.

But this is a very simple change, just change srcu_read_lock() and
srcu_read_unlock() into srcu_read_lock_lite() and srcu_read_unlock_lite().

Would one of your bcachefs early adopters be up for this task?

							Thanx, Paul

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