linux-kernel - Re: [PATCH RFC v2 00/10] SLUB percpu sheaves

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Message-ID: <d731cf1b-a594-983e-90e1-9380410e38fc@gentwo.org>
Date: Fri, 14 Feb 2025 10:28:11 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Vlastimil Babka <vbabka@...e.cz>
cc: Suren Baghdasaryan <surenb@...gle.com>, 
    "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
    David Rientjes <rientjes@...gle.com>, 
    Roman Gushchin <roman.gushchin@...ux.dev>, 
    Hyeonggon Yoo <42.hyeyoo@...il.com>, Uladzislau Rezki <urezki@...il.com>, 
    linux-mm@...ck.org, linux-kernel@...r.kernel.org, rcu@...r.kernel.org, 
    maple-tree@...ts.infradead.org, 
    Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
    Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH RFC v2 00/10] SLUB percpu sheaves

On Fri, 14 Feb 2025, Vlastimil Babka wrote:

> - Cheaper fast paths. For allocations, instead of local double cmpxchg,
>   after Patch 5 it's preempt_disable() and no atomic operations. Same for
>   freeing, which is normally a local double cmpxchg only for a short
>   term allocations (so the same slab is still active on the same cpu when
>   freeing the object) and a more costly locked double cmpxchg otherwise.
>   The downside is the lack of NUMA locality guarantees for the allocated
>   objects.

The local double cmpxchg is not an atomic instruction. For that it would
need a lock prefix.

The local cmpxchg is atomic vs an interrupt because the interrupt can only
occur between instructions. That is true for any processor instruction.

We use the fact that the cmpxchg does a RMV in one unbreakable
instruction to ensure that interrupts cannot do evil things to the fast path.