Re: [PATCHv2 1/2] rcu/tree: handle VM stoppage in stall detection

[Sergey Senozhatsky]

On (21/05/21 14:38), Paul E. McKenney wrote:
> 
> And on that otherwise inexplicable refetch of the jiffies counter within
> check_cpu_stall(), the commit below makes it more effective.
> 
> If check_cpu_stall() is delayed before or while printing the stall
> warning, we really want to wait the full time duration between the
> end of that stall warning and the start of the next one.
>

Nice improvement!

> Of course, if there is some way to learn whether printk() is overloaded,
> even more effective approaches could be taken.

There is no better to do this.

> commit b9c5dc2856c1538ccf2d09246df2b58bede72cca
> Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Date:   Fri May 21 14:23:03 2021 -0700
> 
>     rcu: Start timing stall repetitions after warning complete
>     
>     Systems with low-bandwidth consoles can have very large printk()
>     latencies, and on such systems it makes no sense to have the next RCU CPU
>     stall warning message start output before the prior message completed.
>     This commit therefore sets the time of the next stall only after the
>     prints have completed.  While printing, the time of the next stall
>     message is set to ULONG_MAX/2 jiffies into the future.
>     
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

FWIW,

Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>

> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 05012a8081a1..ff239189a627 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -648,6 +648,7 @@ static void print_cpu_stall(unsigned long gps)
>  
>  static void check_cpu_stall(struct rcu_data *rdp)
>  {
> +	bool didstall = false;
>  	unsigned long gs1;
>  	unsigned long gs2;
>  	unsigned long gps;
> @@ -693,7 +694,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  	    ULONG_CMP_GE(gps, js))
>  		return; /* No stall or GP completed since entering function. */
>  	rnp = rdp->mynode;
> -	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
> +	jn = jiffies + ULONG_MAX / 2;
>  	if (rcu_gp_in_progress() &&
>  	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>  	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
> @@ -710,6 +711,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  		print_cpu_stall(gps);
>  		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
>  			rcu_ftrace_dump(DUMP_ALL);
> +		didstall = true;
>  
>  	} else if (rcu_gp_in_progress() &&
>  		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
> @@ -727,6 +729,11 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  		print_other_cpu_stall(gs2, gps);
>  		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
>  			rcu_ftrace_dump(DUMP_ALL);
> +		didstall = true;
> +	}
> +	if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {

Can `rcu_state.jiffies_stall` change here?

> +		jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
> +		WRITE_ONCE(rcu_state.jiffies_stall, jn);
>  	}
>  }