linux-kernel - Re: Fix MAX_STACK_ENTRIES from 100 to 32

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Date:   Sun, 09 Jul 2023 21:09:14 +0800
From:   "wardenjohn" <ydzhang@...ux.alibaba.com>
To:     "Bagas Sanjaya" <bagasdotme@...il.com>
Cc:     "jpoimboe" <jpoimboe@...nel.org>, "jikos" <jikos@...nel.org>,
        "mbenes" <mbenes@...e.cz>, "pmladek" <pmladek@...e.com>,
        "joe.lawrence" <joe.lawrence@...hat.com>,
        "Kernel Live Patching" <live-patching@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: Fix MAX_STACK_ENTRIES from 100 to 32

OK, I will resubmit the patch by git-send-email(1) instead. :)

But I want ask how can I provide the Link to discussion?
And what is v2 patch?
I am sorry that it is my first time to join the kernel discussion. 

I am looking forward to get the guidance from you. Thanks!

The reason of reducing MAX_STACK_ENTRIES from 100 to 32 is as follows:
In my daily work, I found that all the function stack will not achieve the number of 32.
Therefore, setting the array of 100 may be a waste of kernel memory. Therefore, I suggest
to reduce the number of entries of the stack entries from 100 to 32.

Here is an example of the call trace:
[20409.505602]  [<ffffffff81168861>] group_sched_out+0x61/0xb0
[20409.514791]  [<ffffffff81168bfd>] ctx_sched_out+0xad/0xf0
[20409.520307]  [<ffffffff8116a03d>] __perf_install_in_context+0xbd/0x1b0
[20409.526952]  [<ffffffff811649b0>] remote_function+0x40/0x50
[20409.532644]  [<ffffffff810f1666>] generic_exec_single+0x156/0x1a0
[20409.538864]  [<ffffffff81164970>] ? perf_event_set_output+0x190/0x190
[20409.545425]  [<ffffffff810f170f>] smp_call_function_single+0x5f/0xa0
[20409.551895]  [<ffffffff811f5e70>] ? alloc_file+0xa0/0xf0
[20409.557326]  [<ffffffff81163523>] task_function_call+0x53/0x80
[20409.563274]  [<ffffffff81169f80>] ? perf_cpu_hrtimer_handler+0x1b0/0x1b0
[20409.570089]  [<ffffffff81166688>] perf_install_in_context+0x78/0x120
[20409.576558]  [<ffffffff8116da54>] SYSC_perf_event_open+0x794/0xa40
[20409.582852]  [<ffffffff8116e169>] SyS_perf_event_open+0x9/0x10
[20409.588803]  [<ffffffff8166bf3d>] system_call_fastpath+0x16/0x1b
[20409.594926]  [<ffffffff8166bddd>] ? system_call_after_swapgs+0xca/0x214

------------------------------------------------------------------
From:Bagas Sanjaya <bagasdotme@...il.com>
Send Time:2023年7月9日(星期日) 16:07
To:wardenjohn <ydzhang@...ux.alibaba.com>
Cc:jpoimboe <jpoimboe@...nel.org>; jikos <jikos@...nel.org>; mbenes <mbenes@...e.cz>; pmladek <pmladek@...e.com>; joe.lawrence <joe.lawrence@...hat.com>; Kernel Live Patching <live-patching@...r.kernel.org>; Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject:Re: Fix MAX_STACK_ENTRIES from 100 to 32


On Sat, Jul 08, 2023 at 09:56:34AM +0800, wardenjohn wrote:
> Thanks for reading my suggestion. I found that the array for task stack entries when
> doing livepatch function check is too large which seems to be unnecessary. Therefore,
> I suggest to fix the MAX_STACK_ENTRIES from 100 to 32.

Can you provide Link: to the discussion? Yet, I guess this is somehow
v2 patch.

> 
> The patch is as follows:
> 
> From ee27da5e64daced159257f54170a31141e943710 Mon Sep 17 00:00:00 2001
> From: Yongde Zhang <ydzhang@...ux.alibaba.com>
> Date: Sat, 8 Jul 2023 09:40:50 +0800
> Subject: [PATCH] Fix MAX_STACK_ENTRIES to 32
> 
> When checking the task stack, using an stack array of size 100 
> seems to be to large for a task stack. Therefore, I suggest to
> change the stack size from 100 to 32. 

Why is MAX_STACK_ENTRIES=100 overkill? And why do you reduce it?

> 
> Signed-off-by: Yongde Zhang <ydzhang@...ux.alibaba.com>
> ---
>  kernel/livepatch/transition.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index e54c3d60a904..8d61c62b0c27 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -14,7 +14,7 @@
>  #include "patch.h"
>  #include "transition.h"
>  
> -#define MAX_STACK_ENTRIES  100
> +#define MAX_STACK_ENTRIES  32
>  static DEFINE_PER_CPU(unsigned long[MAX_STACK_ENTRIES], klp_stack_entries);
>  
>  #define STACK_ERR_BUF_SIZE 128 

Your patch is MIME'd, please submit it with git-send-email(1) instead.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara