Re: [PATCH 6/8] mm/vmscan: Centralise timeout values for reclaim_throttle

[NeilBrown]

On Tue, 19 Oct 2021, Mel Gorman wrote:
...
> +	switch(reason) {
> +	case VMSCAN_THROTTLE_NOPROGRESS:
> +	case VMSCAN_THROTTLE_WRITEBACK:
> +		timeout = HZ/10;
> +
> +		if (atomic_inc_return(&pgdat->nr_writeback_throttled) == 1) {
> +			WRITE_ONCE(pgdat->nr_reclaim_start,
> +				node_page_state(pgdat, NR_THROTTLED_WRITTEN));

You have introduced a behaviour change that wasn't flagged in the commit
message.
Previously nr_writeback_throttled was only incremented for
VMSCAN_THROTTLE_WRITEBACK, now it is incremented for
VMSCAN_THROTTLE_NOPROGRESS as well.  

Some justification would be good.

> +		}
> +
> +		break;
> +	case VMSCAN_THROTTLE_ISOLATED:
> +		timeout = HZ/50;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +		timeout = HZ;
> +		break;
>  	}
>  
>  	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
>  	ret = schedule_timeout(timeout);
>  	finish_wait(wqh, &wait);
>  
> -	if (acct_writeback)
> +	if (reason == VMSCAN_THROTTLE_ISOLATED)

(defect) I think you want "!=" there.

While the numbers a still magic, they are now well documented and all in
one place - a definite improvement!

Thanks,
NeilBrown