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Message-ID: <20200419214808.ga3fdpl22fzukawh@master>
Date: Sun, 19 Apr 2020 21:48:08 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Wei Yang <richard.weiyang@...il.com>, josh@...htriplett.org,
rcu@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: simplify the calculation of rcu_state.ncpus
On Sun, Apr 19, 2020 at 08:24:45AM -0700, Paul E. McKenney wrote:
>On Sun, Apr 19, 2020 at 06:02:12AM +0800, Wei Yang wrote:
>> On Sat, Apr 18, 2020 at 11:19 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>> >
>> > On Fri, Apr 17, 2020 at 09:39:51PM +0000, Wei Yang wrote:
>> > > There is only 1 bit set in mask, which means the difference between
>> > > oldmask and the new one would be at the position where the bit is set in
>> > > mask.
>> > >
>> > > Based on this knowledge, rcu_state.ncpus could be calculated by checking
>> > > whether mask is already set in oldmask.
>> >
>> > Nice!!! Good eyes!
>> >
>> > > BTW, the comment at the last of this line is mysterious. Not sure it
>> > > could be removed or not.
>> >
>> > The "^^^" in that comment says to look at the comment on the preceding
>> > line. Memory-ordering functions like smp_store_release() are supposed
>> > to have comments indicating what they are ordering. ;-)
>> >
>> > Could you please do the following things and resubmit?
>> >
>> > 1. Forward-port to -rcu branch dev? This tree lives here:
>> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>> >
>> > 2. Given that oldmask is used only to test to see if a new bit
>> > was set, why not just replace oldmask with a bool variable
>> > that is set to "!(rnp->expmaskinitnext & mask)" before the
>> > bit is ORed into rnp->expmaskinitnext?
>> >
>> > 3. Put the comment inside the "if" statement with the
>> > smp_store_release().
>> >
>> > 4. In -rcu, you will find a ASSERT_EXCLUSIVE_WRITER() statement
>> > that should also be placed inside the "if" statement with
>> > the smp_store_release().
>> >
>>
>> Oops, my email client EAT this mail. Hope this mail will not be banned.
>
>This one made it. ;-)
>
>> I adjust the code a little according to your suggestion like below. Is this
>> what you expected?
>
>Much better, thank you!
>
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index f288477ee1c2..f01367a80b70 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -3732,10 +3732,9 @@ void rcu_cpu_starting(unsigned int cpu)
>> {
>> unsigned long flags;
>> unsigned long mask;
>> - int nbits;
>> - unsigned long oldmask;
>> struct rcu_data *rdp;
>> struct rcu_node *rnp;
>> + bool has_seen;
>
>Could you please use a more descriptive name, perhaps something like
>"newcpu"? Yes, "has_seen" does make sense, but it leaves the reader
>the question "has seen what?".
>
>> if (per_cpu(rcu_cpu_started, cpu))
>> return;
>> @@ -3747,13 +3746,13 @@ void rcu_cpu_starting(unsigned int cpu)
>> mask = rdp->grpmask;
>> raw_spin_lock_irqsave_rcu_node(rnp, flags);
>> WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
>> - oldmask = rnp->expmaskinitnext;
>> + has_seen = rnp->expmaskinitnext & mask;
>> rnp->expmaskinitnext |= mask;
>> - oldmask ^= rnp->expmaskinitnext;
>> - nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
>> - /* Allow lockless access for expedited grace periods. */
>> - smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + nbits); /* ^^^ */
>> - ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
>> + if (!has_seen) {
>> + /* Allow lockless access for expedited grace periods. */
>> + smp_store_release(&rcu_state.ncpus, rcu_state.ncpus +
>> 1); /* ^^^ */
>> + ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
>
>One advantage of flipping the conditional is that it allows dropping the
>"if" statement, which saves a few lines of code and gets closer to your
>original suggestion:
>
> newcpu = !(rnp->expmaskinitnext & mask);
> rnp->expmaskinitnext |= mask;
> /* Allow lockless access for expedited grace periods. */
> smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + newcpu); /* ^^^ */
> ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
>
>Yes, this does execute a store-release that doesn't change anything,
>but this is OK because (1) CPU hotplug is infrequent and (2) CPU hotplug
>has very high overhead. The overhead of the store is thus neglible.
>(But please do not try this to heavily contended cachelines on fastpaths!
I thought I got your point.
>
>And yes, I should have thought of this in response to your initial patch,
>but then again, I should have thought of this when writing this code in
>the first place, shouldn't I have? ;-)
>
Faire enough.
> Thanx, Paul
>
>> + }
>> rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
>> rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
>> rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
>>
>> > Thanx, Paul
>> >
>> > > Signed-off-by: Wei Yang <richard.weiyang@...il.com>
>> > > ---
>> > > kernel/rcu/tree.c | 6 ++----
>> > > 1 file changed, 2 insertions(+), 4 deletions(-)
>> > >
>> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> > > index d91c9156fab2..f0d9251fa663 100644
>> > > --- a/kernel/rcu/tree.c
>> > > +++ b/kernel/rcu/tree.c
>> > > @@ -3364,7 +3364,6 @@ void rcu_cpu_starting(unsigned int cpu)
>> > > {
>> > > unsigned long flags;
>> > > unsigned long mask;
>> > > - int nbits;
>> > > unsigned long oldmask;
>> > > struct rcu_data *rdp;
>> > > struct rcu_node *rnp;
>> > > @@ -3381,10 +3380,9 @@ void rcu_cpu_starting(unsigned int cpu)
>> > > rnp->qsmaskinitnext |= mask;
>> > > oldmask = rnp->expmaskinitnext;
>> > > rnp->expmaskinitnext |= mask;
>> > > - oldmask ^= rnp->expmaskinitnext;
>> > > - nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
>> > > /* Allow lockless access for expedited grace periods. */
>> > > - smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + nbits); /* ^^^ */
>> > > + if (!(oldmask & mask))
>> > > + smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + 1); /* ^^^ */
>> > > rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
>> > > rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
>> > > rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
>> > > --
>> > > 2.23.0
>> > >
--
Wei Yang
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