linux-kernel - Re: [PATCH] vmscan: add a vmscan event for reclaim

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Message-Id: <20241009134900.7cd8fe9000a9fafd7ca1c592@linux-foundation.org>
Date: Wed, 9 Oct 2024 13:49:00 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jaewon Kim <jaewon31.kim@...sung.com>
Cc: sj@...nel.org, minchan@...nel.org, vbabka@...e.cz,
 kaleshsingh@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 jaewon31.kim@...il.com
Subject: Re: [PATCH] vmscan: add a vmscan event for reclaim_pages

On Wed,  9 Oct 2024 18:31:24 +0900 Jaewon Kim <jaewon31.kim@...sung.com> wrote:

> The reclaim_folio_list uses a dummy reclaim_stat and is not being
> used. To know the memory stat, add a new trace event. This is useful how
> how many pages are not reclaimed or why.
> 
> This is an example.
> mm_vmscan_reclaim_pages: nr_scanned=17 nr_reclaimed=17 nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activate_anon=0 nr_activate_file=0 nr_ref_keep=0 nr_unmap_fail=0
> 
> Currenlty reclaim_folio_list is only called by reclaim_pages, and
> reclaim_pages is used by damon and madvise. In the latest Android,
> reclaim_pages is also used by shmem to reclaim all pages in a
> address_space.
> 

This looks like it will add some overhead when tracing has been
enabled.  Has this been measured and is it significant?

Also, we're adding a significant amount of code for a simple trace
record.  Do others think this is justifiable?