> On Feb 28, 2023, at 3:39 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 28, 2023 at 04:51:21PM +0100, Uros Bizjak wrote:
>> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == 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 in
>> front of cmpxchg).
>
> 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?
>
> So try_cmpxchg gives: mov, cmpxchg, cmp, jne
> Where as cmpxchg gives: mov, cmpxchg, mov, jne
>
> 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:
>
> Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Ah there probably is a lesser mov to hold the value to compare against,
but maybe not always?
Would be nice to clarify that in the changelog anyway.
Thanks,
- Joel
>
> thanks,
>
> - Joel
>
>
>>
>> No functional change intended.
>>
>> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
>> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
>> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
>> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
>> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
>> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
>> ---
>> kernel/rcu/tree_stall.h | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> 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 = jiffies + ULONG_MAX / 2;
>> if (rcu_gp_in_progress() &&
>> (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>> - cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>> + try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
>>
>> /*
>> * 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)
>>
>> } else if (rcu_gp_in_progress() &&
>> ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
>> - cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>> + try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
>>
>> /*
>> * If a virtual machine is stopped by the host it can look to
>> --
>> 2.39.2
>>
