Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

[Joel Fernandes]

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>

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
>