Re: [PATCH] mm, memcg: Do not high throttle allocators based on wraparound

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 31, 2020 at 04:24:24PM +0100, Chris Down wrote:
> From: Jakub Kicinski <kuba@kernel.org>
> 
> If a cgroup violates its memory.high constraints, we may end
> up unduly penalising it. For example, for the following hierarchy:
> 
> A:   max high, 20 usage
> A/B: 9 high, 10 usage
> A/C: max high, 10 usage
> 
> We would end up doing the following calculation below when calculating
> high delay for A/B:
> 
> A/B: 10 - 9 = 1...
> A:   20 - PAGE_COUNTER_MAX = 21, so set max_overage to 21.
> 
> This gets worse with higher disparities in usage in the parent.
> 
> I have no idea how this disappeared from the final version of the patch,
> but it is certainly Not Good(tm). This wasn't obvious in testing
> because, for a simple cgroup hierarchy with only one child, the result
> is usually roughly the same. It's only in more complex hierarchies that
> things go really awry (although still, the effects are limited to a
> maximum of 2 seconds in schedule_timeout_killable at a maximum).
> 
> [chris@chrisdown.name: changelog]
> 
> Fixes: e26733e0d0ec ("mm, memcg: throttle allocators based on ancestral memory.high")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@vger.kernel.org # 5.4.x

Oops.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>