On Tue, Mar 22, 2022 at 07:22:48PM +0100, Michal Koutny wrote:
> This was observed with memcontrol selftest/new LTP test but can be also
> reproduced in simplified setup of two siblings:
>
> `parent .low=50M
> ` s1 .low=50M .current=50M+ε
> ` s2 .low=0M .current=50M
>
> The expectation is that s2/memory.events:low will be zero under outer
> reclaimer since no protection should be given to cgroup s2 (even with
> memory_recursiveprot).
>
> However, this does not happen. The apparent reason is that when s1 is
> considered for (proportional) reclaim the scanned proportion is rounded
> up to SWAP_CLUSTER_MAX and slightly over-proportional amount is
> reclaimed. Consequently, when the effective low value of s2 is
> calculated, it observes unclaimed parent's protection from s1
> (ε-SWAP_CLUSTER_MAX in theory) and effectively appropriates it.
> The effect is slightly regularized protection (workload dependent)
> between siblings and misreported MEMCG_LOW event when reclaiming s2 with
> this protection.
>
> Fix the behavior by not reporting breached memory.low in such
> situations. (This affects also setups where all siblings have
> memory.low=0, parent's memory.events:low will still be non-zero when
> parent's memory.low is breached but it will be reduced by the events
> originated in children.)
>
> Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
> Reported-by: Richard Palethorpe <rpalethorpe@xxxxxxxx>
> Link: https://lore.kernel.org/all/20220321101429.3703-1-rpalethorpe@xxxxxxxx/
> Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
Hi Michal!
Does it mean that in the following configuration:
`parent .low=50M
` s1 .low=0M .current=50M
` s2 .low=0M .current=50M
there will be no memory.events::low at all? (assuming the recursive thing is on)
Thanks!
