Re: Patch "mm/damon/core: use number of passed access sampling as a timer" has been added to the 6.6-stable tree

Hi Sasha,

Thank you for picking this patch.

On Thu, 21 Dec 2023 09:53:01 -0500 Sasha Levin sashal@kernel.org wrote:

This is a note to let you know that I've just added the patch titled

mm/damon/core: use number of passed access sampling as a timer

to the 6.6-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-damon-core-use-number-of-passed-access-sampling-a.patch and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

commit dfda8d41e94ee98ebd2ad78c7cb49625a8c92474 Author: SeongJae Park sj@kernel.org Date: Thu Sep 14 02:15:23 2023 +0000

mm/damon/core: use number of passed access sampling as a timer

[ Upstream commit 4472edf63d6630e6cf65e205b4fc8c3c94d0afe5 ]

DAMON sleeps for sampling interval after each sampling, and check if the
aggregation interval and the ops update interval have passed using
ktime_get_coarse_ts64() and baseline timestamps for the intervals.  That
design is for making the operations occur at deterministic timing
regardless of the time that spend for each work.  However, it turned out
it is not that useful, and incur not-that-intuitive results.

After all, timer functions, and especially sleep functions that DAMON uses
to wait for specific timing, are not necessarily strictly accurate.  It is
legal design, so no problem.  However, depending on such inaccuracies, the
nr_accesses can be larger than aggregation interval divided by sampling
interval.  For example, with the default setting (5 ms sampling interval
and 100 ms aggregation interval) we frequently show regions having
nr_accesses larger than 20.  Also, if the execution of a DAMOS scheme
takes a long time, next aggregation could happen before enough number of
samples are collected.  This is not what usual users would intuitively
expect.

Since access check sampling is the smallest unit work of DAMON, using the
number of passed sampling intervals as the DAMON-internal timer can easily
avoid these problems.  That is, convert aggregation and ops update
intervals to numbers of sampling intervals that need to be passed before
those operations be executed, count the number of passed sampling
intervals, and invoke the operations as soon as the specific amount of
sampling intervals passed.  Make the change.

Note that this could make a behavioral change to settings that using
intervals that not aligned by the sampling interval.  For example, if the
sampling interval is 5 ms and the aggregation interval is 12 ms, DAMON
effectively uses 15 ms as its aggregation interval, because it checks
whether the aggregation interval after sleeping the sampling interval.
This change will make DAMON to effectively use 10 ms as aggregation
interval, since it uses 'aggregation interval / sampling interval *
sampling interval' as the effective aggregation interval, and we don't use
floating point types.  Usual users would have used aligned intervals, so
this behavioral change is not expected to make any meaningful impact, so
just make this change.

Link: https://lkml.kernel.org/r/20230914021523.60649-1-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 6376a8245956 ("mm/damon/core: make damon_start() waits until kdamond_fn() starts")

I think adding this patch on 6.6.y has no problem. Nonetheless, Greg notified me the patch that depends on this ("mm/damon/core: make damon_start() waits until kdamond_fn() starts") cannot cleanly applied on 6.1.y and 6.6.y[1,2], and hence I sent conflict-resolved patches for those[3,4] before.

Hence this patch might not really required, but I also think adding this now might help merging future fixes. I don't have strong opinion on whether this patch should be added to 6.6.y or not. I hope you to select a way that better for minimizing stable kernels maintenance overhead.

[1] https://lore.kernel.org/stable/2023121849-ambulance-violate-e5b2@gregkh/ [2] https://lore.kernel.org/stable/2023121843-pension-tactile-868b@gregkh/ [3] https://lore.kernel.org/r/20231218175939.99263-1-sj@kernel.org [4] https://lore.kernel.org/r/20231218175959.99278-1-sj@kernel.org

Thanks, SJ

[...]