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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall
Date:   Tue, 28 Feb 2023 15:56:09 -0500
Message-Id: <944C2939-BE6B-431D-B751-F052A7684844@joelfernandes.org>
References: <Y/5mguXPPqdP3MZF@google.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
In-Reply-To: <Y/5mguXPPqdP3MZF@google.com>
To:     Uros Bizjak <ubizjak@gmail.com>
Precedence: bulk


> On Feb 28, 2023, at 3:39 PM, Joel Fernandes <joel@joelfernandes.org> wrote=
:
>=20
> =EF=BB=BFOn Tue, Feb 28, 2023 at 04:51:21PM +0100, Uros Bizjak wrote:
>> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) =3D=3D old in
>> check_cpu_stall.  x86 CMPXCHG instruction returns success in ZF flag, so
>> this change saves a compare after cmpxchg (and related move instruction i=
n
>> front of cmpxchg).
>=20
> In my codegen, I am not seeing mov instruction before the cmp removed, how=

> can that be? The rax has to be populated with a mov before cmpxchg right?
>=20
> So try_cmpxchg gives: mov, cmpxchg, cmp, jne
> Where as cmpxchg gives: mov, cmpxchg, mov, jne
>=20
> So yeah you got rid of compare, but I am not seeing reduction in moves.
> Either way, I think it is an improvement due to dropping cmp so:
>=20
> Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Ah there probably is a lesser mov to hold the value to compare against,
but maybe not always?=20

Would be nice to clarify that in the changelog anyway.

Thanks,

 - Joel

>=20
> thanks,
>=20
> - Joel
>=20
>=20
>>=20
>> No functional change intended.
>>=20
>> Cc: "Paul E. McKenney" <paulmck@kernel.org>
>> Cc: Frederic Weisbecker <frederic@kernel.org>
>> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
>> Cc: Josh Triplett <josh@joshtriplett.org>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
>> Cc: Joel Fernandes <joel@joelfernandes.org>
>> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
>> ---
>> kernel/rcu/tree_stall.h | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>=20
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index b10b8349bb2a..d81c88e66b42 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -760,7 +760,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>>    jn =3D jiffies + ULONG_MAX / 2;
>>    if (rcu_gp_in_progress() &&
>>        (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>> -        cmpxchg(&rcu_state.jiffies_stall, js, jn) =3D=3D js) {
>> +        try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
>>=20
>>        /*
>>         * If a virtual machine is stopped by the host it can look to
>> @@ -778,7 +778,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>>=20
>>    } else if (rcu_gp_in_progress() &&
>>           ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
>> -           cmpxchg(&rcu_state.jiffies_stall, js, jn) =3D=3D js) {
>> +           try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
>>=20
>>        /*
>>         * If a virtual machine is stopped by the host it can look to
>> --=20
>> 2.39.2
>>=20