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Date:   Thu, 14 Jan 2021 09:04:26 +0000 (UTC)
From:   Christoph Lameter <cl@linux.com>
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To:     Jann Horn <jannh@google.com>
cc:     Vlastimil Babka <vbabka@suse.cz>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: SLUB: percpu partial object count is highly inaccurate, causing
 some memory wastage and maybe also worse tail latencies?
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On Wed, 13 Jan 2021, Jann Horn wrote:

> Some brainstorming:
>
> Maybe you could have an atomic counter in kmem_cache_cpu that tracks
> the number of empty frozen pages that are associated with a specific
> CPU? So the freeing slowpath would do its cmpxchg_double, and if the


The latencies of these functions are so low that any additional counter
will have significant performance impacts. An atomic counter would be waay
out there.

> You could additionally have a plain percpu counter, not tied to the

The performance critical counters are already all per cpu. I enhanced the
percpu subsystem specifically to support latency critical operations in
the fast path of the slab allocators.