linux-kernel - Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

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Date: Wed, 10 Jan 2024 15:01:36 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: andrey.konovalov@...ux.dev,  Andrew Morton <akpm@...ux-foundation.org>,
  Andrey Konovalov <andreyknvl@...il.com>,  Marco Elver <elver@...gle.com>,
  Alexander Potapenko <glider@...gle.com>,  Dmitry Vyukov
 <dvyukov@...gle.com>,  Vlastimil Babka <vbabka@...e.cz>,
  kasan-dev@...glegroups.com,  Evgenii Stepanov <eugenis@...gle.com>,
  linux-mm@...ck.org,  linux-kernel@...r.kernel.org,  Andrey Konovalov
 <andreyknvl@...gle.com>
Subject: Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

Oscar Salvador <osalvador@...e.de> writes:
>> 
>> With this change, multiple users can still look up records in parallel.

That's a severe misunderstanding -- rwlocks always bounce a cache line,
so the parallelism is significantly reduced.

Normally rwlocks are only worth it if your critical region is quite long.

>> 
>> This is preparatory patch for implementing the eviction of stack records
>> from the stack depot.
>> 
>> Reviewed-by: Alexander Potapenko <glider@...gle.com>
>> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>


Has anyone benchmarked this on a high core count machine? It sounds
pretty bad if every lock aquisition starts bouncing a single cache line.

Consider using RCU or similar.

-Andi