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Subject: Re: [PATCH] debugobjects: call debug_objects_mem_init eariler
To:     Qian Cai <cai@gmx.us>, akpm@linux-foundation.org,
        tglx@linutronix.de
Cc:     yang.shi@linux.alibaba.com, arnd@arndb.de,
        linux-kernel@vger.kernel.org
References: <alpine.DEB.2.21.1811222238270.1665@nanos.tec.linutronix.de>
 <20181123043117.992-1-cai@gmx.us>
From:   Waiman Long <longman@redhat.com>
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Organization: Red Hat
Message-ID: <dfa544fc-18d8-f1bf-fadf-46fdbda8d624@redhat.com>
Date:   Thu, 22 Nov 2018 23:59:31 -0500
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Sender: linux-kernel-owner@vger.kernel.org
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On 11/22/2018 11:31 PM, Qian Cai wrote:
> The current value of the early boot static pool size, 1024 is not big
> enough for systems with large number of CPUs with timer or/and workqueue
> objects selected. As the results, systems have 60+ CPUs with both timer
> and workqueue objects enabled could trigger "ODEBUG: Out of memory.
> ODEBUG disabled".
>
> Some debug objects are allocated during the early boot. Enabling some
> options like timers or workqueue objects may increase the size required
> significantly with large number of CPUs. For example,
>
> CONFIG_DEBUG_OBJECTS_TIMERS:
> No. CPUs x 2 (worker pool) objects:
> start_kernel
>   workqueue_init_early
>     init_worker_pool
>       init_timer_key
>         debug_object_init
>
> No. CPUs objects (CONFIG_HIGH_RES_TIMERS):
> sched_init
>   hrtick_rq_init
>     hrtimer_init
>
> CONFIG_DEBUG_OBJECTS_WORK:
> No. CPUs x 6 (workqueue) objects:
> workqueue_init_early
>   alloc_workqueue
>     __alloc_workqueue_key
>       alloc_and_link_pwqs
>         init_pwq
>
> Also, plus No. CPUs objects:
> perf_event_init
>   __init_srcu_struct
>     init_srcu_struct_fields
>       init_srcu_struct_nodes
>         __init_work
>
> However, none of the things are actually used or required beofre
> debug_objects_mem_init() is invoked.
>
> According to tglx,
> "the reason why the call is at this place in start_kernel() is
> historical. It's because back in the days when debugobjects were added
> the memory allocator was enabled way later than today. So we can just
> move the debug_objects_mem_init() call right before sched_init()."
>
> Afterwards, when calling debug_objects_mem_init(), interrupts have
> already been disabled and lockdep_init() will only be called later, so
> no need to worry about interrupts in
> debug_objects_replace_static_objects().
>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Qian Cai <cai@gmx.us>
> ---
>  init/main.c        | 3 ++-
>  lib/debugobjects.c | 8 --------
>  2 files changed, 2 insertions(+), 9 deletions(-)
>
> diff --git a/init/main.c b/init/main.c
> index ee147103ba1b..f2c35dc50851 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -600,6 +600,8 @@ asmlinkage __visible void __init start_kernel(void)
>  	/* trace_printk can be enabled here */
>  	early_trace_init();
>  
> +	debug_objects_mem_init();
> +
>  	/*
>  	 * Set up the scheduler prior starting any interrupts (such as the
>  	 * timer interrupt). Full topology setup happens at smp_init()
> @@ -697,7 +699,6 @@ asmlinkage __visible void __init start_kernel(void)
>  #endif
>  	page_ext_init();
>  	kmemleak_init();
> -	debug_objects_mem_init();
>  	setup_per_cpu_pageset();
>  	numa_policy_init();
>  	acpi_early_init();
> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
> index 70935ed91125..cc5818ced652 100644
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -1132,13 +1132,6 @@ static int __init debug_objects_replace_static_objects(void)
>  		hlist_add_head(&obj->node, &objects);
>  	}
>  
> -	/*
> -	 * When debug_objects_mem_init() is called we know that only
> -	 * one CPU is up, so disabling interrupts is enough
> -	 * protection. This avoids the lockdep hell of lock ordering.
> -	 */
> -	local_irq_disable();

I think you should have a comment saying that debug_objects_mm_init() is
called early with only one CPU up and interrupt disabled. So it is safe
to replace static objects without any protection.

> -
>  	/* Remove the statically allocated objects from the pool */
>  	hlist_for_each_entry_safe(obj, tmp, &obj_pool, node)
>  		hlist_del(&obj->node);
> @@ -1158,7 +1151,6 @@ static int __init debug_objects_replace_static_objects(void)
>  			cnt++;
>  		}
>  	}
> -	local_irq_enable();
>  
>  	pr_debug("%d of %d active objects replaced\n",
>  		 cnt, obj_pool_used);

-Longman