Re: [PATCH 0/6] mm: Remote LRU per-cpu pagevec cache/per-cpu page list drain support

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 21 Sep 2021 18:13:18 +0200 Nicolas Saenz Julienne <nsaenzju@redhat.com> wrote:

> This series introduces an alternative locking scheme around mm/swap.c's per-cpu
> LRU pagevec caches and mm/page_alloc.c's per-cpu page lists which will allow
> for remote CPUs to drain them. Currently, only a local CPU is permitted to
> change its per-cpu lists, and it's expected to do so, on-demand, whenever a
> process demands it (by means of queueing an drain task on the local CPU). Most
> systems will handle this promptly, but it'll cause problems for NOHZ_FULL CPUs
> that can't take any sort of interruption without breaking their functional
> guarantees (latency, bandwidth, etc...). Having a way for these processes to
> remotely drain the lists themselves will make co-existing with isolated CPUs
> possible, at the cost of more constraining locks.
> 
> Fortunately for non-NOHZ_FULL users, the alternative locking scheme and remote
> drain code are conditional to a static key which is disabled by default. This
> guarantees minimal functional or performance regressions. The feature will only
> be enabled if NOHZ_FULL's initialization process was successful.

That all looks pretty straightforward.  Obvious problems are:

- Very little test coverage for the spinlocked code paths.  Virtually
  all test setups will be using local_lock() and the code path you care
  about will go untested.

  I hope that whoever does test the spinlock version will be running
  full debug kernels, including lockdep.  Because adding a spinlock
  where the rest of the code expects local_lock might introduce
  problems.

  A fix for all of this would be to enable the spin_lock code paths
  to be tested more widely.  Perhaps you could add a boot-time kernel
  parameter (or, not as good, a Kconfig thing) which forces the use of
  the spinlock code even on non-NOHZ_FULL systems.

  Or perhaps this debug/testing mode _should_ be enabled by Kconfig,
  so kernel fuzzers sometimes turn it on.

  Please have a think about all of this?

- Maintainability.  Few other MM developers will think about this new
  spinlocked mode much, and they are unlikely to runtime test the
  spinlock mode.  Adding the force-spinlocks-mode-on knob will help
  with this.