[PATCH 0/3 v7] oom: capture unreclaimable slab info in oom message when kernel panic

[PATCH 0/3 v7] oom: capture unreclaimable slab info in oom message when kernel panic

[Yang Shi]

Recently we ran into a oom issue, kernel panic due to no killable process.
The dmesg shows huge unreclaimable slabs used almost 100% memory, but kdump doesn't capture vmcore due to some reason.

So, it may sound better to capture unreclaimable slab info in oom message when kernel panic to aid trouble shooting and cover the corner case.
Since kernel already panic, so capturing more information sounds worthy and doesn't bother normal oom killer.

With the patchset, tools/vm/slabinfo has a new option, "-U", to show unreclaimable slab only.

And, oom will print all non zero (num_objs * size != 0) unreclaimable slabs in oom killer message.

For details, please see the commit log for each commit.

Changelog v6 -> v7:
* Added unreclaim_slabs_oom_ratio proc knob, unreclaimable slabs info will be dumped when unreclaimable slabs amount : all user memory > the ratio

Changelog v5 a??> v6:
* Fixed a checkpatch.pl warning for patch #2

Changelog v4 a??> v5:
* Solved the comments from David
* Build test SLABINFO = n

Changelog v3 a??> v4:
* Solved the comments from David
* Added Davida??s Acked-by in patch 1

Changelog v2 a??> v3:
* Show used size and total size of each kmem cache per Davida??s comment

Changelog v1 a??> v2:
* Removed the original patch 1 (a??mm: slab: output reclaimable flag in /proc/slabinfoa??) since Christoph suggested it might break the compatibility and /proc/slabinfo is legacy
* Added Christopha??s Acked-by
* Removed acquiring slab_mutex per Tetsuoa??s comment


Yang Shi (3):
      tools: slabinfo: add "-U" option to show unreclaimable slabs only
      mm: oom: show unreclaimable slab info when kernel panic
      doc: add description for unreclaim_slabs_oom_ratio

 Documentation/sysctl/vm.txt | 12 ++++++++++++
 include/linux/oom.h         |  1 +
 include/uapi/linux/sysctl.h |  1 +
 kernel/sysctl.c             |  9 +++++++++
 kernel/sysctl_binary.c      |  1 +
 mm/oom_kill.c               | 31 +++++++++++++++++++++++++++++++
 mm/slab.h                   |  8 ++++++++
 mm/slab_common.c            | 29 +++++++++++++++++++++++++++++
 tools/vm/slabinfo.c         | 11 ++++++++++-
 9 files changed, 102 insertions(+), 1 deletion(-)

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[PATCH 1/3] tools: slabinfo: add "-U" option to show unreclaimable slabs only

[Yang Shi]

Add "-U" option to show unreclaimable slabs only.

"-U" and "-S" together can tell us what unreclaimable slabs use the most
memory to help debug huge unreclaimable slabs issue.

Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
---
 tools/vm/slabinfo.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tools/vm/slabinfo.c b/tools/vm/slabinfo.c
index b9d34b3..de8fa11 100644
--- a/tools/vm/slabinfo.c
+++ b/tools/vm/slabinfo.c
@@ -83,6 +83,7 @@ struct aliasinfo {
 int sort_loss;
 int extended_totals;
 int show_bytes;
+int unreclaim_only;
 
 /* Debug options */
 int sanity;
@@ -132,6 +133,7 @@ static void usage(void)
 		"-L|--Loss              Sort by loss\n"
 		"-X|--Xtotals           Show extended summary information\n"
 		"-B|--Bytes             Show size in bytes\n"
+		"-U|--Unreclaim		Show unreclaimable slabs only\n"
 		"\nValid debug options (FZPUT may be combined)\n"
 		"a / A          Switch on all debug options (=FZUP)\n"
 		"-              Switch off all debug options\n"
@@ -568,6 +570,9 @@ static void slabcache(struct slabinfo *s)
 	if (strcmp(s->name, "*") == 0)
 		return;
 
+	if (unreclaim_only && s->reclaim_account)
+		return;
+
 	if (actual_slabs == 1) {
 		report(s);
 		return;
@@ -1346,6 +1351,7 @@ struct option opts[] = {
 	{ "Loss", no_argument, NULL, 'L'},
 	{ "Xtotals", no_argument, NULL, 'X'},
 	{ "Bytes", no_argument, NULL, 'B'},
+	{ "Unreclaim", no_argument, NULL, 'U'},
 	{ NULL, 0, NULL, 0 }
 };
 
@@ -1357,7 +1363,7 @@ int main(int argc, char *argv[])
 
 	page_size = getpagesize();
 
-	while ((c = getopt_long(argc, argv, "aAd::Defhil1noprstvzTSN:LXB",
+	while ((c = getopt_long(argc, argv, "aAd::Defhil1noprstvzTSN:LXBU",
 						opts, NULL)) != -1)
 		switch (c) {
 		case '1':
@@ -1438,6 +1444,9 @@ int main(int argc, char *argv[])
 		case 'B':
 			show_bytes = 1;
 			break;
+		case 'U':
+			unreclaim_only = 1;
+			break;
 		default:
 			fatal("%s: Invalid option '%c'\n", argv[0], optopt);
 
-- 
1.8.3.1

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[PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

Kernel may panic when oom happens without killable process sometimes it
is caused by huge unreclaimable slabs used by kernel.

Although kdump could help debug such problem, however, kdump is not
available on all architectures and it might be malfunction sometime.
And, since kernel already panic it is worthy capturing such information
in dmesg to aid touble shooting.

Print out unreclaimable slab info (used size and total size) which
actual memory usage is not zero (num_objs * size != 0) when:
  - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
  - panic_on_oom is set or no killable process

The output looks like:

Unreclaimable slab info:
Name                      Used          Total
rpc_buffers               31KB         31KB
rpc_tasks                  7KB          7KB
ebitmap_node            1964KB       1964KB
avtab_node              5024KB       5024KB
xfs_buf                 1402KB       1402KB
xfs_ili                  134KB        134KB
xfs_efi_item             115KB        115KB
xfs_efd_item             115KB        115KB
xfs_buf_item             134KB        134KB
xfs_log_item_desc        342KB        342KB
xfs_trans               1412KB       1412KB
xfs_ifork                212KB        212KB

Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
---
 include/linux/oom.h         |  1 +
 include/uapi/linux/sysctl.h |  1 +
 kernel/sysctl.c             |  9 +++++++++
 kernel/sysctl_binary.c      |  1 +
 mm/oom_kill.c               | 31 +++++++++++++++++++++++++++++++
 mm/slab.h                   |  8 ++++++++
 mm/slab_common.c            | 29 +++++++++++++++++++++++++++++
 7 files changed, 80 insertions(+)

diff --git a/include/linux/oom.h b/include/linux/oom.h
index 76aac4c..a732c74 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -105,4 +105,5 @@ extern unsigned long oom_badness(struct task_struct *p,
 extern int sysctl_oom_dump_tasks;
 extern int sysctl_oom_kill_allocating_task;
 extern int sysctl_panic_on_oom;
+extern int sysctl_unreclaim_slabs_oom_ratio;
 #endif /* _INCLUDE_LINUX_OOM_H */
diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
index e13d480..9c4155e 100644
--- a/include/uapi/linux/sysctl.h
+++ b/include/uapi/linux/sysctl.h
@@ -194,6 +194,7 @@ enum
 	VM_PANIC_ON_OOM=33,	/* panic at out-of-memory */
 	VM_VDSO_ENABLED=34,	/* map VDSO into new processes? */
 	VM_MIN_SLAB=35,		 /* Percent pages ignored by zone reclaim */
+	VM_UNRECLAIM_SLABS_OOM_RATIO=36,/* Percent pages dumping unreclaimable slabs when oom */
 };
 
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6648fbb..81a93ee 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1266,6 +1266,15 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
 		.proc_handler	= proc_dointvec,
 	},
 	{
+		.procname	= "unreclaim_slabs_oom_ratio",
+		.data		= &sysctl_unreclaim_slabs_oom_ratio,
+		.maxlen		= sizeof(sysctl_unreclaim_slabs_oom_ratio),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one_hundred,
+	},
+	{
 		.procname	= "overcommit_ratio",
 		.data		= &sysctl_overcommit_ratio,
 		.maxlen		= sizeof(sysctl_overcommit_ratio),
diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
index 58ea8c0..23f1e18 100644
--- a/kernel/sysctl_binary.c
+++ b/kernel/sysctl_binary.c
@@ -170,6 +170,7 @@ struct bin_table {
 	{ CTL_INT,	VM_PANIC_ON_OOM,		"panic_on_oom" },
 	{ CTL_INT,	VM_VDSO_ENABLED,		"vdso_enabled" },
 	{ CTL_INT,	VM_MIN_SLAB,			"min_slab_ratio" },
+	{ CTL_INT,	VM_UNRECLAIM_SLABS_OOM_RATIO,	"unreclaim_slabs_oom_ratio" },
 
 	{}
 };
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 99736e0..6359edb 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -43,6 +43,7 @@
 
 #include <asm/tlb.h>
 #include "internal.h"
+#include "slab.h"
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/oom.h>
@@ -50,6 +51,8 @@
 int sysctl_panic_on_oom;
 int sysctl_oom_kill_allocating_task;
 int sysctl_oom_dump_tasks = 1;
+/* unreclaimable slabs is 50% of all LRU pages */
+int sysctl_unreclaim_slabs_oom_ratio = 50;
 
 DEFINE_MUTEX(oom_lock);
 
@@ -160,6 +163,30 @@ static bool oom_unkillable_task(struct task_struct *p,
 	return false;
 }
 
+/*
+ * Print out unreclaimble slabs info unconditionally when
+ * sysctl_unreclaim_slabs_oom_ratio is 0. Otherwise when
+ * unreclaimable slabs : all LRU pages > sysctl_unreclaim_slabs_oom_ratio.
+ */
+static bool is_dump_unreclaim_slabs(void)
+{
+	unsigned long nr_lru;
+
+	nr_lru = global_node_page_state(NR_ACTIVE_ANON) +
+		 global_node_page_state(NR_INACTIVE_ANON) +
+		 global_node_page_state(NR_ACTIVE_FILE) +
+		 global_node_page_state(NR_INACTIVE_FILE) +
+		 global_node_page_state(NR_ISOLATED_ANON) +
+		 global_node_page_state(NR_ISOLATED_FILE) +
+		 global_node_page_state(NR_UNEVICTABLE);
+
+	if (sysctl_unreclaim_slabs_oom_ratio > 0)
+		return (global_node_page_state(NR_SLAB_UNRECLAIMABLE) * 100 /
+			nr_lru) > (unsigned long)sysctl_unreclaim_slabs_oom_ratio;
+	else
+		return true;
+}
+
 /**
  * oom_badness - heuristic function to determine which candidate task to kill
  * @p: task struct of which task we should calculate
@@ -960,6 +987,8 @@ static void check_panic_on_oom(struct oom_control *oc,
 	if (is_sysrq_oom(oc))
 		return;
 	dump_header(oc, NULL);
+	if (is_dump_unreclaim_slabs())
+		dump_unreclaimable_slab();
 	panic("Out of memory: %s panic_on_oom is enabled\n",
 		sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
 }
@@ -1044,6 +1073,8 @@ bool out_of_memory(struct oom_control *oc)
 	/* Found nothing?!?! Either we hang forever, or we panic. */
 	if (!oc->chosen && !is_sysrq_oom(oc) && !is_memcg_oom(oc)) {
 		dump_header(oc, NULL);
+		if (is_dump_unreclaim_slabs())
+			dump_unreclaimable_slab();
 		panic("Out of memory and no killable processes...\n");
 	}
 	if (oc->chosen && oc->chosen != (void *)-1UL) {
diff --git a/mm/slab.h b/mm/slab.h
index 0733628..b0496d1 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -505,6 +505,14 @@ static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
 void memcg_slab_stop(struct seq_file *m, void *p);
 int memcg_slab_show(struct seq_file *m, void *p);
 
+#ifdef CONFIG_SLABINFO
+void dump_unreclaimable_slab(void);
+#else
+static inline void dump_unreclaimable_slab(void)
+{
+}
+#endif
+
 void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr);
 
 #ifdef CONFIG_SLAB_FREELIST_RANDOM
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 904a83b..d08213d 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1272,6 +1272,35 @@ static int slab_show(struct seq_file *m, void *p)
 	return 0;
 }
 
+void dump_unreclaimable_slab(void)
+{
+	struct kmem_cache *s, *s2;
+	struct slabinfo sinfo;
+
+	pr_info("Unreclaimable slab info:\n");
+	pr_info("Name                      Used          Total\n");
+
+	/*
+	 * Here acquiring slab_mutex is unnecessary since we don't prefer to
+	 * get sleep in oom path right before kernel panic, and avoid race
+	 * condition.
+	 * Since it is already oom, so there should be not any big allocation
+	 * which could change the statistics significantly.
+	 */
+	list_for_each_entry_safe(s, s2, &slab_caches, list) {
+		if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))
+			continue;
+
+		memset(&sinfo, 0, sizeof(sinfo));
+		get_slabinfo(s, &sinfo);
+
+		if (sinfo.num_objs > 0)
+			pr_info("%-17s %10luKB %10luKB\n", cache_name(s),
+				(sinfo.active_objs * s->size) / 1024,
+				(sinfo.num_objs * s->size) / 1024);
+	}
+}
+
 #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
 void *memcg_slab_start(struct seq_file *m, loff_t *pos)
 {
-- 
1.8.3.1

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Christopher Lameter]

On Wed, 27 Sep 2017, Yang Shi wrote:

> Print out unreclaimable slab info (used size and total size) which
> actual memory usage is not zero (num_objs * size != 0) when:
>   - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
>   - panic_on_oom is set or no killable process

Ok. I like this much more than the earlier releases.

> diff --git a/mm/slab.h b/mm/slab.h
> index 0733628..b0496d1 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -505,6 +505,14 @@ static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
>  void memcg_slab_stop(struct seq_file *m, void *p);
>  int memcg_slab_show(struct seq_file *m, void *p);
>
> +#ifdef CONFIG_SLABINFO
> +void dump_unreclaimable_slab(void);
> +#else
> +static inline void dump_unreclaimable_slab(void)
> +{
> +}
> +#endif

CONFIG_SLABINFO? How does this relate to the oom info? /proc/slabinfo
support is optional. Oom info could be included even if CONFIG_SLABINFO
goes away. Remove the #ifdef?

Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

On 9/27/17 12:14 AM, Christopher Lameter wrote:
> On Wed, 27 Sep 2017, Yang Shi wrote:
> 
>> Print out unreclaimable slab info (used size and total size) which
>> actual memory usage is not zero (num_objs * size != 0) when:
>>    - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
>>    - panic_on_oom is set or no killable process
> 
> Ok. I like this much more than the earlier releases.
> 
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 0733628..b0496d1 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -505,6 +505,14 @@ static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
>>   void memcg_slab_stop(struct seq_file *m, void *p);
>>   int memcg_slab_show(struct seq_file *m, void *p);
>>
>> +#ifdef CONFIG_SLABINFO
>> +void dump_unreclaimable_slab(void);
>> +#else
>> +static inline void dump_unreclaimable_slab(void)
>> +{
>> +}
>> +#endif
> 
> CONFIG_SLABINFO? How does this relate to the oom info? /proc/slabinfo
> support is optional. Oom info could be included even if CONFIG_SLABINFO
> goes away. Remove the #ifdef?

Because we want to dump the unreclaimable slab info in oom info.

Thanks,
Yang

> 

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Christopher Lameter]

On Thu, 28 Sep 2017, Yang Shi wrote:
> > CONFIG_SLABINFO? How does this relate to the oom info? /proc/slabinfo
> > support is optional. Oom info could be included even if CONFIG_SLABINFO
> > goes away. Remove the #ifdef?
>
> Because we want to dump the unreclaimable slab info in oom info.

CONFIG_SLABINFO and /proc/slabinfo have nothing to do with the
unreclaimable slab info.

Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

On 9/27/17 2:59 PM, Christopher Lameter wrote:
> On Thu, 28 Sep 2017, Yang Shi wrote:
>>> CONFIG_SLABINFO? How does this relate to the oom info? /proc/slabinfo
>>> support is optional. Oom info could be included even if CONFIG_SLABINFO
>>> goes away. Remove the #ifdef?
>>
>> Because we want to dump the unreclaimable slab info in oom info.
> 
> CONFIG_SLABINFO and /proc/slabinfo have nothing to do with the
> unreclaimable slab info.

The current design uses "struct slabinfo" and get_slabinfo() to retrieve 
some info, i.e. active objs, etc. They are protected by CONFIG_SLABINFO.

We could replicate the logic in get_slabinfo without using struct 
slabinfo, but it sounds not that necessary and CONFIG_SLABINFO is 
typically enabled by default and it is not shown in menuconfig.

Thanks,
Yang

> 

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Christopher Lameter]

On Thu, 28 Sep 2017, Yang Shi wrote:

> > CONFIG_SLABINFO and /proc/slabinfo have nothing to do with the
> > unreclaimable slab info.
>
> The current design uses "struct slabinfo" and get_slabinfo() to retrieve some
> info, i.e. active objs, etc. They are protected by CONFIG_SLABINFO.

Ok I guess then those need to be moved out of CONFIG_SLABINFO. Otherwise
dumping of slabs will not be supported when disabling that option.

Or dump CONFIG_SLABINFO ..

Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

On 9/30/17 11:43 PM, Christopher Lameter wrote:
> On Thu, 28 Sep 2017, Yang Shi wrote:
> 
>>> CONFIG_SLABINFO and /proc/slabinfo have nothing to do with the
>>> unreclaimable slab info.
>>
>> The current design uses "struct slabinfo" and get_slabinfo() to retrieve some
>> info, i.e. active objs, etc. They are protected by CONFIG_SLABINFO.
> 
> Ok I guess then those need to be moved out of CONFIG_SLABINFO. Otherwise
> dumping of slabs will not be supported when disabling that option.

Yes.

> 
> Or dump CONFIG_SLABINFO ..

I prefer to this. It sounds pointless to keep CONFIG_SLABINFO. It is 
always on by default and can't be changed in menuconfig.

Thanks,
Yang

> 

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Michal Hocko]

On Wed 27-09-17 08:53:35, Yang Shi wrote:
> Kernel may panic when oom happens without killable process sometimes it
> is caused by huge unreclaimable slabs used by kernel.
> 
> Although kdump could help debug such problem, however, kdump is not
> available on all architectures and it might be malfunction sometime.
> And, since kernel already panic it is worthy capturing such information
> in dmesg to aid touble shooting.
> 
> Print out unreclaimable slab info (used size and total size) which
> actual memory usage is not zero (num_objs * size != 0) when:
>   - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
>   - panic_on_oom is set or no killable process

OK, this is better but I do not see why this should be tunable via proc.
Can we start with simple NR_SLAB_UNRECLAIMABLE > LRU_PAGES and place it
into dump_header so that we get the report also during regular OOM?
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

On 9/27/17 3:45 AM, Michal Hocko wrote:
> On Wed 27-09-17 08:53:35, Yang Shi wrote:
>> Kernel may panic when oom happens without killable process sometimes it
>> is caused by huge unreclaimable slabs used by kernel.
>>
>> Although kdump could help debug such problem, however, kdump is not
>> available on all architectures and it might be malfunction sometime.
>> And, since kernel already panic it is worthy capturing such information
>> in dmesg to aid touble shooting.
>>
>> Print out unreclaimable slab info (used size and total size) which
>> actual memory usage is not zero (num_objs * size != 0) when:
>>    - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
>>    - panic_on_oom is set or no killable process
> 
> OK, this is better but I do not see why this should be tunable via proc.

Just thought someone might want to dump unreclaimable slab info 
unconditionally.

> Can we start with simple NR_SLAB_UNRECLAIMABLE > LRU_PAGES and place it
> into dump_header so that we get the report also during regular OOM
Yes.

Thanks,
Yang

> 

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Michal Hocko]

On Thu 28-09-17 01:25:50, Yang Shi wrote:
> 
> 
> On 9/27/17 3:45 AM, Michal Hocko wrote:
> > On Wed 27-09-17 08:53:35, Yang Shi wrote:
> > > Kernel may panic when oom happens without killable process sometimes it
> > > is caused by huge unreclaimable slabs used by kernel.
> > > 
> > > Although kdump could help debug such problem, however, kdump is not
> > > available on all architectures and it might be malfunction sometime.
> > > And, since kernel already panic it is worthy capturing such information
> > > in dmesg to aid touble shooting.
> > > 
> > > Print out unreclaimable slab info (used size and total size) which
> > > actual memory usage is not zero (num_objs * size != 0) when:
> > >    - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
> > >    - panic_on_oom is set or no killable process
> > 
> > OK, this is better but I do not see why this should be tunable via proc.
> 
> Just thought someone might want to dump unreclaimable slab info
> unconditionally.

If that ever happens then we will eventually add it. But do not add proc
knobs for theoretical usecases. We will have to maintain them and it
can turn into a maint. pain. Like some others in the past.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 2/3] mm: oom: show unreclaimable slab info when kernel panic

[Yang Shi]

On 10/2/17 12:26 AM, Michal Hocko wrote:
> On Thu 28-09-17 01:25:50, Yang Shi wrote:
>>
>>
>> On 9/27/17 3:45 AM, Michal Hocko wrote:
>>> On Wed 27-09-17 08:53:35, Yang Shi wrote:
>>>> Kernel may panic when oom happens without killable process sometimes it
>>>> is caused by huge unreclaimable slabs used by kernel.
>>>>
>>>> Although kdump could help debug such problem, however, kdump is not
>>>> available on all architectures and it might be malfunction sometime.
>>>> And, since kernel already panic it is worthy capturing such information
>>>> in dmesg to aid touble shooting.
>>>>
>>>> Print out unreclaimable slab info (used size and total size) which
>>>> actual memory usage is not zero (num_objs * size != 0) when:
>>>>     - unreclaimable slabs : all user memory > unreclaim_slabs_oom_ratio
>>>>     - panic_on_oom is set or no killable process
>>>
>>> OK, this is better but I do not see why this should be tunable via proc.
>>
>> Just thought someone might want to dump unreclaimable slab info
>> unconditionally.
> 
> If that ever happens then we will eventually add it. But do not add proc
> knobs for theoretical usecases. We will have to maintain them and it
> can turn into a maint. pain. Like some others in the past.

It has been removed since v8. Currently the only condition is 
unreclaimable slabs > user memory.

Thanks,
Yang

> 

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[PATCH 3/3] doc: add description for unreclaim_slabs_oom_ratio

[Yang Shi]

Add the description for unreclaim_slabs_oom_ratio in
Documentation/sysctl/vm.txt.

Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
---
 Documentation/sysctl/vm.txt | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 9baf66a..29926e3 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -59,6 +59,7 @@ Currently, these files are in /proc/sys/vm:
 - stat_interval
 - stat_refresh
 - swappiness
+- unreclaim_slabs_oom_ratio
 - user_reserve_kbytes
 - vfs_cache_pressure
 - watermark_scale_factor
@@ -804,6 +805,17 @@ The default value is 60.
 
 ==============================================================
 
+unreclaim_slabs_oom_ratio
+
+The percentage of total unreclaimable slabs amount vs all user memory amount
+(LRU pages). When the real ratio is greater than the value, oom killer would
+dump unreclaimable slabs info when kernel panic.
+The range is 0 - 100. 0 means dump unreclaimable slabs info unconditionally.
+
+The default value is 50.
+
+==============================================================
+
 - user_reserve_kbytes
 
 When overcommit_memory is set to 2, "never overcommit" mode, reserve
-- 
1.8.3.1

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