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Date: Sat, 14 Jan 2017 11:48:06 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...icios.com, josh@...htriplett.org,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
Lance Roy <ldr709@...il.com>
Subject: Re: [PATCH tip/core/rcu 1/3] srcu: More efficient reader counts.
On Sat, Jan 14, 2017 at 10:31:15AM +0100, Ingo Molnar wrote:
>
> Noticed a few minor nits:
And thank you for the review and comments!
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>
> > From: Lance Roy <ldr709@...il.com>
> >
> > SRCU uses two per-cpu counters: a nesting counter to count the number of
> > active critical sections, and a sequence counter to ensure that the nesting
> > counters don't change while they are being added together in
> > srcu_readers_active_idx_check().
> >
> > This patch instead uses per-cpu lock and unlock counters. Because the both
> > counters only increase and srcu_readers_active_idx_check() reads the unlock
> > counter before the lock counter, this achieves the same end without having
> > to increment two different counters in srcu_read_lock(). This also saves a
> > smp_mb() in srcu_readers_active_idx_check().
>
> typo:
>
> s/Because the both counters
> Because both counters
Fixed!
> > A possible problem with this patch is that it can only handle
> > ULONG_MAX - NR_CPUS simultaneous readers, whereas the old version could
> > handle up to ULONG_MAX.
>
> I don't think this is a problem! :-)
Here is hoping! ;-)
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Signed-off-by: Lance Roy <ldr709@...il.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > ---
> > include/linux/srcu.h | 4 +-
> > kernel/rcu/rcutorture.c | 18 +++++++-
> > kernel/rcu/srcu.c | 117 ++++++++++++++++++------------------------------
> > 3 files changed, 62 insertions(+), 77 deletions(-)
> >
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index dc8eb63c6568..0caea34d8c5f 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -34,8 +34,8 @@
> > #include <linux/workqueue.h>
> >
> > struct srcu_struct_array {
> > - unsigned long c[2];
> > - unsigned long seq[2];
> > + unsigned long lock_count[2];
> > + unsigned long unlock_count[2];
> > };
> >
> > struct rcu_batch {
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 87c51225ceec..6e4fd7680c70 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -564,10 +564,24 @@ static void srcu_torture_stats(void)
> > pr_alert("%s%s per-CPU(idx=%d):",
> > torture_type, TORTURE_FLAG, idx);
> > for_each_possible_cpu(cpu) {
> > + unsigned long l0, l1;
> > + unsigned long u0, u1;
> > long c0, c1;
> > + struct srcu_struct_array* counts =
> > + per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
>
> Please don't break the line to pacify checkpatch - if the line is too long then
> maybe split out the loop body into a helper function - but keeping it a bit longer
> than 80 cols is fine as well.
Creating a helper function woujld leave me several characters over still,
so I just created the long line. Another approach would be to split the
definition and the initialization into two statements, but that would
add a line.
> > - c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
> > - c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
> > + u0 = counts->unlock_count[!idx];
> > + u1 = counts->unlock_count[idx];
> > +
> > + /* Make sure that a lock is always counted if the corresponding
> > + unlock is counted. */
> > + smp_rmb();
>
> That's not the standard kernel code comment style.
That is embarrassing! Fixed.
> > +
> > + l0 = counts->lock_count[!idx];
> > + l1 = counts->lock_count[idx];
> > +
> > + c0 = (long)(l0 - u0);
> > + c1 = (long)(l1 - u1);
>
> These type casts look unnecessary to me.
Indeed, given that we are assigning to a long rather than just computing.
> > for_each_possible_cpu(cpu) {
> > - t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
> > + struct srcu_struct_array* cpu_counts =
> > + per_cpu_ptr(sp->per_cpu_ref, cpu);
> > + t = READ_ONCE(cpu_counts->lock_count[idx]);
> > sum += t;
>
>
> > for_each_possible_cpu(cpu) {
> > - t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
> > + struct srcu_struct_array* cpu_counts =
> > + per_cpu_ptr(sp->per_cpu_ref, cpu);
> > + t = READ_ONCE(cpu_counts->unlock_count[idx]);
> > sum += t;
>
> These linebreak look ugly as well. Some abbreviation of types and variables might
> help:
>
> s/srcu_struct_array/srcu_array
> s/cpu_counts/cpuc
>
> ?
Why not? Fixed. ;-)
> > + * If the locks are the same as the unlocks, then there must of have
> > + * been no readers on this index at some time in between. This does not
> > + * mean that there are no more readers, as one could have read the
> > + * current index but have incremented the lock counter yet.
> >
> > + * Note that there can be at most NR_CPUS worth of readers using the old
> > + * index that haven't incremented ->lock_count[] yet. Therefore, the
> > + * sum of the ->lock_count[]s cannot increment enough times to overflow
> > + * and end up equal the sum of the ->unlock_count[]s, as long as there
> > + * are at most ULONG_MAX - NR_CPUS readers at a time. (Yes, this does
> > + * mean that systems having more than a billion or so CPUs need to be
> > + * 64-bit systems.) Therefore, the only way that the return values of
> > + * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
> > + * are no active readers using this index.
>
> typo:
>
> s/must of have been no readers/
> must have been no readers
>
> Also, maybe I'm misreading it, but shouldn't it be:
>
> s/as one could have read the current index but have incremented the lock counter yet.
> /as one could have read the current index but not have incremented the lock counter yet.
>
> ?
Agreed on both, fixed.
> Also, the title:
>
> srcu: More efficient reader counts.
>
> should have a verb and no full stop, i.e. something like:
>
> srcu: Implement more efficient reader counts
And this one as well.
Thanx, Paul
> Thanks,
>
> Ingo
>
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