Re: [PATCH 1/2] mm/damon/sysfs: Implement recording feature

[cuiyangpei]

On Sun, Jan 28, 2024 at 08:28:04AM -0800, SeongJae Park wrote:
> On Sun, 28 Jan 2024 17:13:00 +0800 cuiyangpei <cuiyangpei@xxxxxxxxx> wrote:
> 
> > On Fri, Jan 26, 2024 at 12:04:54AM -0800, SeongJae Park wrote:
> [...]
> > > So, 'update_schemes_tried_regions' command is firstly handled by
> > > 'damon_sysfs_cmd_request_callback()', which is registered as
> > > after_wmarks_check() and after_aggregation() callback.  Hence
> > > 'update_schemes_tried_regions' command is still effectively working in
> > > aggregation interval granularity.  I think this is what you found, right?
> > > 
> > Yes.
> > > If I'm not wrongly understanding your point, I think the concern is valid.  I
> > > think we should make it works in sampling interval granularity.  I will try to
> > > make so.  Would that work for your use case?
> > > 
> > It's much better than working in aggregation interval.
> 
> Thank you for confirming.  I will start working on this.
> 

Great, looking forward to seeing the progress.

> > 
> > I have a question. Why does the 'update_schemes_tried_regions' command need to work
> > in the sampling time or aggregation time? 'update_schemes_tried_regions' is a
> > relatively special state that updates the regions that corresponding operation scheme.
> > Can it be separated from other states and controlled by sysfs node to respond immediately
> > after being written?
> 
> Mainly because the region data is updated by a kdamond thread.  To safely
> access the region, the accessor should do some kind of synchronization with the
> kdamond thread.  To minimize such synchronization overhead, DAMON let the API
> users (kernel components) to register callbacks which kdamond invokes under
> specific events including 'after_sampling' or 'after_aggregate'.  Because the
> callback is executed in the kdamond thread, callbacks can safely access the
> data without additional synchronization.  DAMON sysfs interface is using the
> callback mechanism, and hence need to work in the sampling or aggregation
> times.
> 
Thank you for the detailed explanation.

> Thanks,
> SJ
> 
> [...]