Re: [PATCH] mm/vmscan: add balance_pgdat begin/end tracepoints

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Apr 24, 2026 at 08:46:24AM +0800, SUVONOV BUNYOD wrote:
> Thank you for reviewing Shakeel,
> 
> > Do we need to trace highest_zoneidx at the end? Can it change within
> > balance_pgdat()?
> 
> highest_zoneidx does not change within a balance_pgdat() invocation. It
> is passed in as an argument and remains the classzone bound used for the
> balancing checks throughout the function.
> 
> I kept highest_zoneidx in the end tracepoint to make the outcome event
> self-contained. In principle, begin/end correlation is possible, but
> under sustained memory pressure kswapd reclaim can be frequent enough
> that consumers may prefer to analyze end events directly, and any
> dependence on matching begin/end becomes less convenient and less robust
> in the presence of filtering or dropped trace records.
> 
> Since nr_reclaimed and the final order are only known at the end, having
> highest_zoneidx there allows end-only analysis without correlating with
> the begin event.
> 
> For example, it lets users answer questions like:
> - this pass reclaimed too much or too little memory; what highest_zoneidx
> did that result correspond to?
> - how much reclaim was done when balancing up to ZONE_NORMAL vs other
> classzone bounds?
> - when highest_zoneidx == ZONE_NORMAL, how often did reclaim finish at
> order=0?
> 
> So it is there because it provides context for the end-of-reclaim result.
> Do you think this is sufficient justification? If not, then I can drop it
> from the end tracepoint in v2.

I think it is ok but let's add this reasoning in the commit message.