linux-kernel - Re: [PATCH v4] vmstat: Kernel stack usage histogram

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Message-Id: <20240723234600.d05817293ec1eb2d4ad87be7@linux-foundation.org>
Date: Tue, 23 Jul 2024 23:46:00 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: jpoimboe@...nel.org, kent.overstreet@...ux.dev, peterz@...radead.org,
 nphamcs@...il.com, cerasuolodomenico@...il.com, surenb@...gle.com,
 lizhijian@...itsu.com, willy@...radead.org, shakeel.butt@...ux.dev,
 vbabka@...e.cz, ziy@...dia.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v4] vmstat: Kernel stack usage histogram

On Thu, 18 Jul 2024 20:26:11 +0000 Pasha Tatashin <pasha.tatashin@...een.com> wrote:

> As part of the dynamic kernel stack project, we need to know the amount
> of data that can be saved by reducing the default kernel stack size [1].
> 
> Provide a kernel stack usage histogram to aid in optimizing kernel stack
> sizes and minimizing memory waste in large-scale environments. The
> histogram divides stack usage into power-of-two buckets and reports the
> results in /proc/vmstat. This information is especially valuable in
> environments with millions of machines, where even small optimizations
> can have a significant impact.

x86_64 allmodconfig:

In file included from <command-line>:
In function 'init_memcg_events',
    inlined from 'mem_cgroup_css_alloc' at mm/memcontrol.c:3616:3:
././include/linux/compiler_types.h:510:45: error: call to '__compiletime_assert_2305' declared with attribute error: BUILD_BUG_ON failed: NR_VM_EVENT_ITEMS >= S8_MAX
  510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                             ^
././include/linux/compiler_types.h:491:25: note: in definition of macro '__compiletime_assert'
  491 |                         prefix ## suffix();                             \
      |                         ^~~~~~
././include/linux/compiler_types.h:510:9: note: in expansion of macro '_compiletime_assert'
  510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |         ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |         ^~~~~~~~~~~~~~~~
mm/memcontrol.c:444:9: note: in expansion of macro 'BUILD_BUG_ON'
  444 |         BUILD_BUG_ON(NR_VM_EVENT_ITEMS >= S8_MAX);
      |         ^~~~~~~~~~~~

This looks legitimate - is it time to switch to int16_t?