Re: [PATCH v3 2/2] mm: memcg: introduce new event to trace shrink_memcg

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 29 Nov 2023 18:10:33 +0100
Michal Hocko <mhocko@suse.com> wrote:

> > > How? You are only reporting the number of reclaimed pages and no
> > > reclaimed pages could be not just because of low/min limits but
> > > generally because of other reasons. You would need to report also the
> > > number of scanned/isolated pages.
> > >    
> > 
> > From my perspective, if memory control group (memcg) protection
> > restrictions occur, we can identify them by the absence of the end()
> > pair of begin(). Other reasons will have both tracepoints raised.  
> 
> That is not really great way to detect that TBH. Trace events could be
> lost and then you simply do not know what has happened.

Note, you can detect dropped events. If there's a dropped event, you can
ignore the "missing end" from a beginning. You could also make synthetic
events that pair an end event with a beginning event (which uses the last
begin event found). Synthetic event creation is not affected by dropped
events.

There's a lot you can to get information with the prospect of dropped
events. I would not use that as rationale for not using events.

-- Steve