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

[Jens Axboe <axboe@kernel.dk>]

On 1/31/23 8:48?AM, David Hildenbrand wrote:
> 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.

Sure, I can move that bit into the commit message.

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

That would be preferable in general, but I don't think it's worth
rebasing the series just for that.

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

Thanks! Will add in conjunction with updating the commit message.

-- 
Jens Axboe