Re: [PATCH v13 2/3] mm: Fix OOM killer inaccuracy on large many-core systems

Hi,
sorry to jump in this late but the timing of previous versions didn't
really work well for me.

On Sun 11-01-26 14:49:57, Mathieu Desnoyers wrote:
[...]

Here is a (possibly incomplete) list of the prior approaches that were
used or proposed, along with their downside:

1) Per-thread rss tracking: large error on many-thread processes.

2) Per-CPU counters: up to 12% slower for short-lived processes and 9%
   increased system time in make test workloads [1]. Moreover, the
   inaccuracy increases with O(n^2) with the number of CPUs.

3) Per-NUMA-node counters: requires atomics on fast-path (overhead),
   error is high with systems that have lots of NUMA nodes (32 times
   the number of NUMA nodes).

The approach proposed here is to replace this by the hierarchical
per-cpu counters, which bounds the inaccuracy based on the system
topology with O(N*logN).

The concept of hierarchical pcp counter is interesting and I am
definitely not opposed if there are more users that would benefit.

From the OOM POV, IIUC the primary problem is that get_mm_counter
(percpu_counter_read_positive) is too imprecise on systems when the task
is moving around a large number of cpus. In the list of alternative
solutions I do not see percpu_counter_sum_positive to be mentioned.
oom_badness() is a really slow path and taking the slow path to
calculate a much more precise value seems acceptable. Have you
considered that option?

-- 
Michal Hocko
SUSE Labs