Re: [patch -mm 7/9 v2] oom: replace sysctls with quick mode

[Nick Piggin <npiggin@suse.de>]

On Mon, Feb 15, 2010 at 02:20:18PM -0800, David Rientjes wrote:
> Two VM sysctls, oom dump_tasks and oom_kill_allocating_task, were
> implemented for very large systems to avoid excessively long tasklist
> scans.  The former suppresses helpful diagnostic messages that are
> emitted for each thread group leader that are candidates for oom kill
> including their pid, uid, vm size, rss, oom_adj value, and name; this
> information is very helpful to users in understanding why a particular
> task was chosen for kill over others.  The latter simply kills current,
> the task triggering the oom condition, instead of iterating through the
> tasklist looking for the worst offender.
> 
> Both of these sysctls are combined into one for use on the aforementioned
> large systems: oom_kill_quick.  This disables the now-default
> oom_dump_tasks and kills current whenever the oom killer is called.
> 
> The oom killer rewrite is the perfect opportunity to combine both sysctls
> into one instead of carrying around the others for years to come for
> nothing else than legacy purposes.

I just don't understand this either. There appears to be simply no
performance or maintainability reason to change this.

> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  Documentation/sysctl/vm.txt |   44 +++++-------------------------------------
>  include/linux/oom.h         |    3 +-
>  kernel/sysctl.c             |   13 ++---------
>  mm/oom_kill.c               |    9 +++----
>  4 files changed, 14 insertions(+), 55 deletions(-)
> 
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -43,9 +43,8 @@ Currently, these files are in /proc/sys/vm:
>  - nr_pdflush_threads
>  - nr_trim_pages         (only if CONFIG_MMU=n)
>  - numa_zonelist_order
> -- oom_dump_tasks
>  - oom_forkbomb_thres
> -- oom_kill_allocating_task
> +- oom_kill_quick
>  - overcommit_memory
>  - overcommit_ratio
>  - page-cluster
> @@ -470,27 +469,6 @@ this is causing problems for your system/application.
>  
>  ==============================================================
>  
> -oom_dump_tasks
> -
> -Enables a system-wide task dump (excluding kernel threads) to be
> -produced when the kernel performs an OOM-killing and includes such
> -information as pid, uid, tgid, vm size, rss, cpu, oom_adj score, and
> -name.  This is helpful to determine why the OOM killer was invoked
> -and to identify the rogue task that caused it.
> -
> -If this is set to zero, this information is suppressed.  On very
> -large systems with thousands of tasks it may not be feasible to dump
> -the memory state information for each one.  Such systems should not
> -be forced to incur a performance penalty in OOM conditions when the
> -information may not be desired.
> -
> -If this is set to non-zero, this information is shown whenever the
> -OOM killer actually kills a memory-hogging task.
> -
> -The default value is 0.
> -
> -==============================================================
> -
>  oom_forkbomb_thres
>  
>  This value defines how many children with a seperate address space a specific
> @@ -511,22 +489,12 @@ The default value is 1000.
>  
>  ==============================================================
>  
> -oom_kill_allocating_task
> -
> -This enables or disables killing the OOM-triggering task in
> -out-of-memory situations.
> -
> -If this is set to zero, the OOM killer will scan through the entire
> -tasklist and select a task based on heuristics to kill.  This normally
> -selects a rogue memory-hogging task that frees up a large amount of
> -memory when killed.
> -
> -If this is set to non-zero, the OOM killer simply kills the task that
> -triggered the out-of-memory condition.  This avoids the expensive
> -tasklist scan.
> +oom_kill_quick
>  
> -If panic_on_oom is selected, it takes precedence over whatever value
> -is used in oom_kill_allocating_task.
> +When enabled, this will always kill the task that triggered the oom killer, i.e.
> +the task that attempted to allocate memory that could not be found.  It also
> +suppresses the tasklist dump to the kernel log whenever the oom killer is
> +called.  Typically set on systems with an extremely large number of tasks.
>  
>  The default value is 0.
>  
> diff --git a/include/linux/oom.h b/include/linux/oom.h
> --- a/include/linux/oom.h
> +++ b/include/linux/oom.h
> @@ -63,8 +63,7 @@ static inline void oom_killer_enable(void)
>  }
>  /* for sysctl */
>  extern int sysctl_panic_on_oom;
> -extern int sysctl_oom_kill_allocating_task;
> -extern int sysctl_oom_dump_tasks;
> +extern int sysctl_oom_kill_quick;
>  extern int sysctl_oom_forkbomb_thres;
>  
>  #endif /* __KERNEL__*/
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -941,16 +941,9 @@ static struct ctl_table vm_table[] = {
>  		.proc_handler	= proc_dointvec,
>  	},
>  	{
> -		.procname	= "oom_kill_allocating_task",
> -		.data		= &sysctl_oom_kill_allocating_task,
> -		.maxlen		= sizeof(sysctl_oom_kill_allocating_task),
> -		.mode		= 0644,
> -		.proc_handler	= proc_dointvec,
> -	},
> -	{
> -		.procname	= "oom_dump_tasks",
> -		.data		= &sysctl_oom_dump_tasks,
> -		.maxlen		= sizeof(sysctl_oom_dump_tasks),
> +		.procname	= "oom_kill_quick",
> +		.data		= &sysctl_oom_kill_quick,
> +		.maxlen		= sizeof(sysctl_oom_kill_quick),
>  		.mode		= 0644,
>  		.proc_handler	= proc_dointvec,
>  	},
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -32,9 +32,8 @@
>  #include <linux/security.h>
>  
>  int sysctl_panic_on_oom;
> -int sysctl_oom_kill_allocating_task;
> -int sysctl_oom_dump_tasks;
>  int sysctl_oom_forkbomb_thres = DEFAULT_OOM_FORKBOMB_THRES;
> +int sysctl_oom_kill_quick;
>  static DEFINE_SPINLOCK(zone_scan_lock);
>  
>  /*
> @@ -402,7 +401,7 @@ static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
>  	dump_stack();
>  	mem_cgroup_print_oom_info(mem, p);
>  	show_mem();
> -	if (sysctl_oom_dump_tasks)
> +	if (!sysctl_oom_kill_quick)
>  		dump_tasks(mem);
>  }
>  
> @@ -609,9 +608,9 @@ static void __out_of_memory(gfp_t gfp_mask, int order, unsigned long totalpages,
>  	struct task_struct *p;
>  	unsigned int points;
>  
> -	if (sysctl_oom_kill_allocating_task)
> +	if (sysctl_oom_kill_quick)
>  		if (!oom_kill_process(current, gfp_mask, order, 0, totalpages,
> -			NULL, "Out of memory (oom_kill_allocating_task)"))
> +			NULL, "Out of memory (quick mode)"))
>  			return;
>  retry:
>  	/*

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