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Date:   Mon, 9 Jul 2018 18:08:41 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     David Rientjes <rientjes@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, vmacache: hash addresses based on pmd
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On Mon, 9 Jul 2018 17:50:03 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:

> When perf profiling a wide variety of different workloads, it was found
> that vmacache_find() had higher than expected cost: up to 0.08% of cpu
> utilization in some cases.  This was found to rival other core VM
> functions such as alloc_pages_vma() with thp enabled and default
> mempolicy, and the conditionals in __get_vma_policy().
> 
> VMACACHE_HASH() determines which of the four per-task_struct slots a vma
> is cached for a particular address.  This currently depends on the pfn,
> so pfn 5212 occupies a different vmacache slot than its neighboring
> pfn 5213.
> 
> vmacache_find() iterates through all four of current's vmacache slots
> when looking up an address.  Hashing based on pfn, an address has
> ~1/VMACACHE_SIZE chance of being cached in the first vmacache slot, or
> about 25%, *if* the vma is cached.
> 
> This patch hashes an address by its pmd instead of pte to optimize for
> workloads with good spatial locality.  This results in a higher
> probability of vmas being cached in the first slot that is checked:
> normally ~70% on the same workloads instead of 25%.

Was the improvement quantifiable?

Surprised.  That little array will all be in CPU cache and that loop
should execute pretty quickly?  If it's *that* sensitive then let's zap
the no-longer-needed WARN_ON.  And we could hide all the event counting
behind some developer-only ifdef.

Did you consider LRU-sorting the array instead?