linux-kernel - Re: [PATCH v8 00/16] move per-vma lock into vm_area

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Message-ID: <20250109115142.GC2981@noisy.programming.kicks-ass.net>
Date: Thu, 9 Jan 2025 12:51:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com,
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz,
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	linux-doc@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct

On Wed, Jan 08, 2025 at 06:30:09PM -0800, Suren Baghdasaryan wrote:
> Back when per-vma locks were introduces, vm_lock was moved out of
> vm_area_struct in [1] because of the performance regression caused by
> false cacheline sharing. Recent investigation [2] revealed that the
> regressions is limited to a rather old Broadwell microarchitecture and
> even there it can be mitigated by disabling adjacent cacheline
> prefetching, see [3].
> Splitting single logical structure into multiple ones leads to more
> complicated management, extra pointer dereferences and overall less
> maintainable code. When that split-away part is a lock, it complicates
> things even further. With no performance benefits, there are no reasons
> for this split. Merging the vm_lock back into vm_area_struct also allows
> vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset.
> This patchset:
> 1. moves vm_lock back into vm_area_struct, aligning it at the cacheline
> boundary and changing the cache to be cacheline-aligned to minimize
> cacheline sharing;
> 2. changes vm_area_struct initialization to mark new vma as detached until
> it is inserted into vma tree;
> 3. replaces vm_lock and vma->detached flag with a reference counter;
> 4. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their
> reuse and to minimize call_rcu() calls.

Does not clean up that reattach nonsense :-(