Re: [PATCH v3] printk: Add console owner and waiter logic to load balance console writes

[Vlastimil Babka]

On 11/02/2017 06:45 PM, Steven Rostedt wrote:
...>  	__DEVKMSG_LOG_BIT_ON = 0,
>  	__DEVKMSG_LOG_BIT_OFF,
> @@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility
>  		 * semaphore.  The release will print out buffers and wake up
>  		 * /dev/kmsg and syslog() users.
>  		 */
> -		if (console_trylock())
> +		if (console_trylock()) {
>  			console_unlock();
> +		} else {
> +			struct task_struct *owner = NULL;
> +			bool waiter;
> +			bool spin = false;
> +
> +			printk_safe_enter_irqsave(flags);
> +
> +			raw_spin_lock(&console_owner_lock);
> +			owner = READ_ONCE(console_owner);
> +			waiter = READ_ONCE(console_waiter);
> +			if (!waiter && owner && owner != current) {
> +				WRITE_ONCE(console_waiter, true);
> +				spin = true;
> +			}
> +			raw_spin_unlock(&console_owner_lock);
> +
> +			/*
> +			 * If there is an active printk() writing to the
> +			 * consoles, instead of having it write our data too,
> +			 * see if we can offload that load from the active
> +			 * printer, and do some printing ourselves.
> +			 * Go into a spin only if there isn't already a waiter
> +			 * spinning, and there is an active printer, and
> +			 * that active printer isn't us (recursive printk?).
> +			 */
> +			if (spin) {
> +				/* We spin waiting for the owner to release us */
> +				spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> +				/* Owner will clear console_waiter on hand off */
> +				while (!READ_ONCE(console_waiter))

This should not be negated, right? We should spin while it's true, not
false.

> +					cpu_relax();
> +
> +				spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> +				printk_safe_exit_irqrestore(flags);
> +
> +				/*
> +				 * The owner passed the console lock to us.
> +				 * Since we did not spin on console lock, annotate
> +				 * this as a trylock. Otherwise lockdep will
> +				 * complain.
> +				 */
> +				mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
> +				console_unlock();
> +				printk_safe_enter_irqsave(flags);
> +			}
> +			printk_safe_exit_irqrestore(flags);
> +
> +		}
>  	}