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Subject: Re: [PATCH 0/6] mm: Remote LRU per-cpu pagevec cache/per-cpu page
 list drain support
To:     Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        akpm@linux-foundation.org, frederic@kernel.org
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        tglx@linutronix.de, cl@linux.com, peterz@infradead.org,
        juri.lelli@redhat.com, mingo@redhat.com, mtosatti@redhat.com,
        nilal@redhat.com, mgorman@suse.de, ppandit@redhat.com,
        williams@redhat.com, bigeasy@linutronix.de,
        anna-maria@linutronix.de, linux-rt-users@vger.kernel.org
References: <20210921161323.607817-1-nsaenzju@redhat.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <f608b4bf-aa36-b0c4-e748-4f39010f3d06@suse.cz>
Date:   Tue, 21 Sep 2021 19:59:51 +0200
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On 9/21/21 6:13 PM, Nicolas Saenz Julienne 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.
> 
> This work is based on a previous series by Thomas Gleixner, Anna-Maria
> Gleixner, and Sebastian Andrzej Siewior[1].

These days the pcplist protection is done by local_lock, which solved
the RT concerns. Probably a stupid/infeasible idea, but maybe what you
want to achieve could be more generally solved at the local_lock level?
That on NOHZ_FULL CPUs, local_locks could have this mode where they
could synchronize with remote cpus?

> [1] https://patchwork.kernel.org/project/linux-mm/patch/20190424111208.24459-3-bigeasy@linutronix.de/
> 
> Nicolas Saenz Julienne (6):
>   mm/swap: Introduce lru_cpu_needs_drain()
>   mm/swap: Introduce alternative per-cpu LRU cache locking
>   mm/swap: Allow remote LRU cache draining
>   mm/page_alloc: Introduce alternative per-cpu list locking
>   mm/page_alloc: Allow remote per-cpu page list draining
>   sched/isolation: Enable 'remote_pcpu_cache_access' on NOHZ_FULL
>     systems
> 
>  kernel/sched/isolation.c |   9 +-
>  mm/internal.h            |   2 +
>  mm/page_alloc.c          | 111 ++++++++++++++++-----
>  mm/swap.c                | 202 ++++++++++++++++++++++++++++++---------
>  4 files changed, 253 insertions(+), 71 deletions(-)
>