Re: [PATCH v2 1/2] mm, memcg: reclaim more aggressively before high allocator throttling

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jul 13, 2020 at 12:42:35PM +0100, Chris Down wrote:
> In Facebook production, we've seen cases where cgroups have been put
> into allocator throttling even when they appear to have a lot of slack
> file caches which should be trivially reclaimable.
> 
> Looking more closely, the problem is that we only try a single cgroup
> reclaim walk for each return to usermode before calculating whether or
> not we should throttle. This single attempt doesn't produce enough
> pressure to shrink for cgroups with a rapidly growing amount of file
> caches prior to entering allocator throttling.
> 
> As an example, we see that threads in an affected cgroup are stuck in
> allocator throttling:
> 
>     # for i in $(cat cgroup.threads); do
>     >     grep over_high "/proc/$i/stack"
>     > done
>     [<0>] mem_cgroup_handle_over_high+0x10b/0x150
>     [<0>] mem_cgroup_handle_over_high+0x10b/0x150
>     [<0>] mem_cgroup_handle_over_high+0x10b/0x150
> 
> ...however, there is no I/O pressure reported by PSI, despite a lot of
> slack file pages:
> 
>     # cat memory.pressure
>     some avg10=78.50 avg60=84.99 avg300=84.53 total=5702440903
>     full avg10=78.50 avg60=84.99 avg300=84.53 total=5702116959
>     # cat io.pressure
>     some avg10=0.00 avg60=0.00 avg300=0.00 total=78051391
>     full avg10=0.00 avg60=0.00 avg300=0.00 total=78049640
>     # grep _file memory.stat
>     inactive_file 1370939392
>     active_file 661635072
> 
> This patch changes the behaviour to retry reclaim either until the
> current task goes below the 10ms grace period, or we are making no
> reclaim progress at all. In the latter case, we enter reclaim throttling
> as before.
> 
> To a user, there's no intuitive reason for the reclaim behaviour to
> differ from hitting memory.high as part of a new allocation, as opposed
> to hitting memory.high because someone lowered its value. As such this
> also brings an added benefit: it unifies the reclaim behaviour between
> the two.
> 
> There's precedent for this behaviour: we already do reclaim retries when
> writing to memory.{high,max}, in max reclaim, and in the page allocator
> itself.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Michal Hocko <mhocko@kernel.org>

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