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From: Nick Piggin on 16 Feb 2010 01:30
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(a)jp.fujitsu.com>
> Signed-off-by: David Rientjes <rientjes(a)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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From: David Rientjes on 16 Feb 2010 04:00
On Tue, 16 Feb 2010, Nick Piggin 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.
>

When oom_dump_tasks() is always emitted for out of memory conditions as my
patch does, then these two tunables have the exact same audience: users
with large systems that have extremely long tasklists. They want to avoid
tasklist scanning (either to select a bad process to kill or dump their
information) in oom conditions and simply kill the allocating task. I
chose to combine the two: we're not concerned about breaking the
oom_dump_tasks ABI since it's now the default behavior and since we scan
the tasklist for mempolicy-constrained ooms, users may now choose to
enable oom_kill_allocating_task when they previously wouldn't have. To do
that, they can either use the old sysctl or convert to this new sysctl
with the benefit that we've removed one unnecessary sysctl from
/proc/sys/vm.

As far as I know, oom_kill_allocating_task is only used by SGI, anyway,
since they are the ones who asked for it when I implemented cpuset
tasklist scanning. It's certainly not widely used and since the semantics
for mempolicies have changed, oom_kill_quick may find more users.
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From: David Rientjes on 16 Feb 2010 04:10
On Tue, 16 Feb 2010, KOSAKI Motohiro wrote:

> > > "_quick" is always bad sysctl name.
> >
> > Why? It does exactly what it says: it kills current without doing an
> > expensive tasklist scan and suppresses the possibly long tasklist dump.
> > That's the oom killer's "quick mode."
>
> Because, an administrator think "_quick" implies "please use it always".
> plus, "quick" doesn't describe clealy meanings. oom_dump_tasks does.
>

The audience for both of these tunables (now that oom_dump_tasks is
default to enabled) is users with extremely long tasklists that want to
avoid those scans, so oom_kill_quick implies that it won't waste any time
and will act how it's documented: simply kill current and move on.
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