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

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Message-ID: <CA+CK2bB2fiAGHpD5dsSdxCgPjx7OBra5K8Vn_a4wytPa2U6UjA@mail.gmail.com>
Date: Wed, 24 Jul 2024 10:43:59 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Andrew Morton <akpm@...ux-foundation.org>
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 Wed, Jul 24, 2024 at 2:46 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> 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?

I am looking into this, and will also uninline stack_not_used() and
kstack_histogram() as discussed earlier in the thread.

Pasha