Hi Sasha, Thank you for picking this patch. On Thu, 21 Dec 2023 09:53:01 -0500 Sasha Levin <sashal@xxxxxxxxxx> 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;a=summary > > 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@xxxxxxxxxxxxxxx> know about it. > > > > commit dfda8d41e94ee98ebd2ad78c7cb49625a8c92474 > Author: SeongJae Park <sj@xxxxxxxxxx> > 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@xxxxxxxxxx > Signed-off-by: SeongJae Park <sj@xxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > 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@xxxxxxxxxx [4] https://lore.kernel.org/r/20231218175959.99278-1-sj@xxxxxxxxxx Thanks, SJ [...]
