Re: [PATCH] mm: move FOLL_PIN debug accounting under CONFIG_DEBUG_VM

[David Hildenbrand <david@redhat.com>]

On 31.01.23 16:36, Jens Axboe wrote:
> Using FOLL_PIN for mapping user pages caused a performance regression of
> about 2.7%. Looking at profiles, we see:
> 
> +2.71%  [kernel.vmlinux]  [k] mod_node_page_state
> 
> which wasn't there before. The node page state counters are percpu, but
> with a very low threshold. On my setup, every 108th update ends up
> needing to punt to two atomic_lond_add()'s, which is causing this above
> regression.
> 
> As these counters are purely for debug purposes, move them under
> CONFIG_DEBUG_VM rather than do them unconditionally.
> 
> Fixes: fd20d0c1852e ("block: convert bio_map_user_iov to use iov_iter_extract_pages")
> Fixes: 920756a3306a ("block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages")
> Link: https://lore.kernel.org/linux-block/f57ee72f-38e9-6afa-182f-2794638eadcb@kernel.dk/
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> I added fixes tags, even though it's not a strict fix for this commits.
> But it does fix a performance regression introduced by those commits.
> It's a useful hint for backporting.

I'd just mention them in the commit log instead, but I don't 
particularly care here as long as the commit ID's are stable.

If still possible, I'd include this as a preparational change for these 
commits instead. Anyhow

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb