Re:Re:[PATCH] mm:vmscan: fix inaccurate reclaim during proactive reclaim

Efly Young
2023-07-12T07:44:42Z

>> With commit f53af4285d77 ("mm: vmscan: fix extreme overreclaim
>> and swap floods"), proactive reclaim still seems inaccurate.
>>
>> Our problematic scene also are almost anon pages. Request 1G
>> by writing memory.reclaim will reclaim 1.7G or other values
>> more than 1G by swapping.
>>
>> This try to fix the inaccurate reclaim problem.
>
> I can see how this happens. Direct and kswapd reclaim have much
> smaller nr_to_reclaim targets, so it's less noticable when we loop a
> few times. Proactive reclaim can come in with a rather large value.
>
> What does the reproducer setup look like? Are you calling reclaim on a
> higher level cgroup with several children? Or is the looping coming
> from having multiple zones alone?

Thank you for your comment. The process in a leaf cgroup without children
just malloc 20G anonymous memory and sleep, then calling reclaim in the
leaf cgroup. Before commit f53af4285d77 ("mm: vmscan: fix extreme
overreclaim and swap floods"), reclaimer may reclaim many times the amount
of request. Now it should eventually reclaim in [request, 2 * request).

>> Signed-off-by: Efly Young <yangyifei03@kuaishou.com>
>> ---
>> mm/vmscan.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 9c1c5e8b..2aea8d9 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -6208,7 +6208,7 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>> unsigned long nr_to_scan;
>> enum lru_list lru;
>> unsigned long nr_reclaimed = 0;
>> - unsigned long nr_to_reclaim = sc->nr_to_reclaim;
>> + unsigned long nr_to_reclaim = (sc->nr_to_reclaim - sc->nr_reclaimed);
>
> This can underflow. shrink_list() eats SWAP_CLUSTER_MAX batches out of
> lru_pages >> priority, and only checks reclaimed > to_reclaim
> after. This will then disable the bailout mechanism entirely.
>
> In general, I'm not sure this is the best spot to fix the problem:
>
> - During reclaim/compaction, should_continue_reclaim() may decide that
> more reclaim is required before compaction can proceed. But the
> second cycle might not do anything now, since you remember the work
> done by the previous one.
>
> - shrink_node_memcgs() might do the full batch against the first
> cgroup and not touch the second one anymore. This will result in
> super lopsided behavior when you target a tree of multiple groups.
>
> There might be other spots that break, I haven't checked.
>
> You could go through them one by one, of course. But the truth is,
> larger reclaim targets are the rare exception. Trying to support them
> at the risk of breaking all other reclaim users seems ill-advised.

I agree with your view. These explanations are more considerate. Thank
you again for helping me out.

> A better approach might be to just say: "don't call reclaim with large
> numbers". Have proactive reclaim code handle the batching into smaller
> chunks:
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e8ca4bdcb03c..4b016806dcc7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6696,7 +6696,7 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> lru_add_drain_all();
>
> reclaimed = try_to_free_mem_cgroup_pages(memcg,
> - nr_to_reclaim - nr_reclaimed,
> + min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> GFP_KERNEL, reclaim_options);
>
> if (!reclaimed && !nr_retries--)

May be this way could solve the inaccurate proactive reclaim
problem without breaking the original balance. But may be less
efficient than before?