Re: [PATCH v2 1/3] writeback: Let trace_balance_dirty_pages() take struct dtc as parameter

[Steven Rostedt]

On Tue,  4 Mar 2025 19:03:16 +0800
Tang Yizhou <yizhou.tang@xxxxxxxxxx> wrote:

> @@ -664,16 +660,16 @@ TRACE_EVENT(balance_dirty_pages,
>  	),
>  
>  	TP_fast_assign(
> -		unsigned long freerun = (thresh + bg_thresh) / 2;
> +		unsigned long freerun = (dtc->thresh + dtc->bg_thresh) / 2;
>  		strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
>  
>  		__entry->limit		= global_wb_domain.dirty_limit;
>  		__entry->setpoint	= (global_wb_domain.dirty_limit +
>  						freerun) / 2;
> -		__entry->dirty		= dirty;
> +		__entry->dirty		= dtc->dirty;
>  		__entry->bdi_setpoint	= __entry->setpoint *
> -						bdi_thresh / (thresh + 1);
> -		__entry->bdi_dirty	= bdi_dirty;
> +						dtc->wb_thresh / (dtc->thresh + 1);
> +		__entry->bdi_dirty	= dtc->wb_dirty;
>  		__entry->dirty_ratelimit = KBps(dirty_ratelimit);
>  		__entry->task_ratelimit	= KBps(task_ratelimit);
>  		__entry->dirtied	= dirtied;

I don't know how much of a fast path these tracepoints are in, but instead
of doing the divisions above, why not just save the values in the ring
buffer, and do the divisions in the TP_printk() section, which is done when
the user reads it and not when the code is executing?

-- Steve