EP-2DAD0AFA905A4ACB804C4F82A001242F
Hi Andrew,
Presently in oom_kill.c we calculate badness score of the victim task as per the present RSS counter value of the task.
RSS counter value for any task is usually '[Private (Dirty/Clean)] + [Shared (Dirty/Clean)]' of the task.
We have encountered a situation where values for Private fields are less but value for Shared fields are more and hence make total RSS counter value large. Later on oom situation killing task with highest RSS value but as Private field values are not large hence memory gain after killing this process is not as per the expectation.
For e.g. take below use-case scenario, in which 3 process are running in system.
All these process done mmap for file exist in present directory and then copying data from this file to local allocated pointers in while(1) loop with some sleep. Out of 3 process, 2 process has mmaped file with MAP_SHARED setting and one has mapped file with MAP_PRIVATE setting.
I have all 3 processes in background and checks RSS/PSS value from user space utility (utility over cat /proc/pid/smaps)
Before OOM, below is the consumed memory status for these 3 process (all processes run with oom_score_adj = 0)
====================================================
Comm : 1prg, Pid : 213 (values in kB)
Rss Shared Private Pss
Process : 375764 194596 181168 278460
====================================================
Comm : 3prg, Pid : 217 (values in kB)
Rss Shared Private Pss
Process : 305760 32 305728 305738
====================================================
Comm : 2prg, Pid : 218 (values in kB)
Rss Shared Private Pss
Process : 389980 194596 195384 292676
====================================================
Thus as per present code design, first it would select process [2prg : 218] as bulkiest process as its RSS value is highest to kill. But if we kill this process then only ~195MB would be free as compare to expected ~389MB.
Thus identifying the task based on RSS value is not accurate design and killing that identified process didn?t release expected memory back to system.
We need to calculate victim task based on PSS instead of RSS as PSS value calculates as
PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
For above use-case scenario also, it can be checked that process [3prg : 217] is having largest PSS value and by killing this process we can gain maximum memory (~305MB) as compare to killing process identified based on RSS value.
--
Regards,
Yogesh Gaur.????{.n?+???????+%?????ݶ??w??{.n?+????{??G?????{ay? ʇڙ?,j??f???h?????????z_??(?階?ݢj"???m??????G????????????&???~???iO???z??v?^?m???? ????????I?
2015-05-08 13:29 GMT+08:00 Yogesh Narayan Gaur <[email protected]>:
>
> EP-2DAD0AFA905A4ACB804C4F82A001242F
> Hi Andrew,
>
> Presently in oom_kill.c we calculate badness score of the victim task as per the present RSS counter value of the task.
> RSS counter value for any task is usually '[Private (Dirty/Clean)] + [Shared (Dirty/Clean)]' of the task.
> We have encountered a situation where values for Private fields are less but value for Shared fields are more and hence make total RSS counter value large. Later on oom situation killing task with highest RSS value but as Private field values are not large hence memory gain after killing this process is not as per the expectation.
>
> For e.g. take below use-case scenario, in which 3 process are running in system.
> All these process done mmap for file exist in present directory and then copying data from this file to local allocated pointers in while(1) loop with some sleep. Out of 3 process, 2 process has mmaped file with MAP_SHARED setting and one has mapped file with MAP_PRIVATE setting.
> I have all 3 processes in background and checks RSS/PSS value from user space utility (utility over cat /proc/pid/smaps)
> Before OOM, below is the consumed memory status for these 3 process (all processes run with oom_score_adj = 0)
> ====================================================
> Comm : 1prg, Pid : 213 (values in kB)
> Rss Shared Private Pss
> Process : 375764 194596 181168 278460
> ====================================================
> Comm : 3prg, Pid : 217 (values in kB)
> Rss Shared Private Pss
> Process : 305760 32 305728 305738
> ====================================================
> Comm : 2prg, Pid : 218 (values in kB)
> Rss Shared Private Pss
> Process : 389980 194596 195384 292676
> ====================================================
>
> Thus as per present code design, first it would select process [2prg : 218] as bulkiest process as its RSS value is highest to kill. But if we kill this process then only ~195MB would be free as compare to expected ~389MB.
> Thus identifying the task based on RSS value is not accurate design and killing that identified process didn’t release expected memory back to system.
>
> We need to calculate victim task based on PSS instead of RSS as PSS value calculates as
> PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
> For above use-case scenario also, it can be checked that process [3prg : 217] is having largest PSS value and by killing this process we can gain maximum memory (~305MB) as compare to killing process identified based on RSS value.
>
> --
> Regards,
> Yogesh Gaur.
Great,
in fact, i also encounter this scenario,
i use USS (page map counter == 1) pages
to decide which process should be killed,
seems have the same result as you use PSS,
but PSS is better , it also consider shared pages,
in case some process have large shared pages mapping
but little Private page mapping
BRs,
Yalin
EP-2DAD0AFA905A4ACB804C4F82A001242F
------- Original Message -------
Sender : yalin wang<[email protected]>
Date : May 08, 2015 13:17 (GMT+05:30)
Title : Re: [EDT] oom_killer: find bulkiest task based on pss value
2015-05-08 13:29 GMT+08:00 Yogesh Narayan Gaur :
>>
>> EP-2DAD0AFA905A4ACB804C4F82A001242F
>> Hi Andrew,
>>
>> Presently in oom_kill.c we calculate badness score of the victim task as per the present RSS counter value of the task.
>> RSS counter value for any task is usually '[Private (Dirty/Clean)] + [Shared (Dirty/Clean)]' of the task.
>> We have encountered a situation where values for Private fields are less but value for Shared fields are more and hence make total RSS counter value large. Later on oom situation killing task with highest RSS value but as Private field values are not large hence memory gain after killing this process is not as per the expectation.
>>
>> For e.g. take below use-case scenario, in which 3 process are running in system.
>> All these process done mmap for file exist in present directory and then copying data from this file to local allocated pointers in while(1) loop with some sleep. Out of 3 process, 2 process has mmaped file with MAP_SHARED setting and one has mapped file with MAP_PRIVATE setting.
>> I have all 3 processes in background and checks RSS/PSS value from user space utility (utility over cat /proc/pid/smaps)
>> Before OOM, below is the consumed memory status for these 3 process (all processes run with oom_score_adj = 0)
>> ====================================================
>> Comm : 1prg, Pid : 213 (values in kB)
>> Rss Shared Private Pss
>> Process : 375764 194596 181168 278460
>> ====================================================
>> Comm : 3prg, Pid : 217 (values in kB)
>> Rss Shared Private Pss
>> Process : 305760 32 305728 305738
>> ====================================================
>> Comm : 2prg, Pid : 218 (values in kB)
>> Rss Shared Private Pss
>> Process : 389980 194596 195384 292676
>> ====================================================
>>
>> Thus as per present code design, first it would select process [2prg : 218] as bulkiest process as its RSS value is highest to kill. But if we kill this process then only ~195MB would be free as compare to expected ~389MB.
>> Thus identifying the task based on RSS value is not accurate design and killing that identified process didn?t release expected memory back to system.
>>
>> We need to calculate victim task based on PSS instead of RSS as PSS value calculates as
>> PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
>> For above use-case scenario also, it can be checked that process [3prg : 217] is having largest PSS value and by killing this process we can gain maximum memory (~305MB) as compare to killing process identified based on RSS value.
>>
>> --
>> Regards,
>> Yogesh Gaur.
>
>Great,
>
> in fact, i also encounter this scenario,
> I use USS (page map counter == 1) pages
> to decide which process should be killed,
> seems have the same result as you use PSS,
> but PSS is better , it also consider shared pages,
> in case some process have large shared pages mapping
> but little Private page mapping
>
> BRs,
> Yalin
I have made patch which identifies bulkiest task on basis of PSS value. Please check below patch.
This patch is correcting the way victim task gets identified in oom condition.
==================
>From 1c3d7f552f696bdbc0126c8e23beabedbd80e423 Mon Sep 17 00:00:00 2001
From: Yogesh Gaur <[email protected]>
Date: Thu, 7 May 2015 01:52:13 +0530
Subject: [PATCH] oom: find victim task based on pss
This patch is identifying bulkiest task to kill by OOM on the basis of PSS value
instead of present RSS values.
There can be scenario where task with highest RSS counter is consuming lot of shared
memory and killing that task didn't release expected amount of memory to system.
PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
RSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean)]
Thus, using PSS value instead of RSS value as PSS value closely matches with actual
memory usage by the task.
This patch is using smaps_pte_range() interface defined in CONFIG_PROC_PAGE_MONITOR.
For case when CONFIG_PROC_PAGE_MONITOR disabled, this simply returns RSS value count.
Signed-off-by: Yogesh Gaur <[email protected]>
Signed-off-by: Amit Arora <[email protected]>
Reviewed-by: Ajeet Yadav <[email protected]>
---
fs/proc/task_mmu.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
include/linux/mm.h | 9 +++++++++
mm/oom_kill.c | 9 +++++++--
3 files changed, 63 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 956b75d..dd962ff 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -964,6 +964,53 @@ struct pagemapread {
bool v2;
};
+/**
+ * get_mm_pss - function to determine PSS count of pages being used for proc p
+ * PSS value=[Private(Dirty/Clean)] + [Shared(Dirty/Clean)/no. of shared task]
+ * @p: task struct of which task we should calculate
+ * @mm: mm struct of the task.
+ *
+ * This function needs to be called under task_lock for calling task 'p'.
+ */
+long get_mm_pss(struct task_struct *p, struct mm_struct *mm)
+{
+ long pss = 0;
+ struct vm_area_struct *vma = NULL;
+ struct mem_size_stats mss;
+ struct mm_walk smaps_walk = {
+ .pmd_entry = smaps_pte_range,
+ .private = &mss,
+ };
+
+ if (mm == NULL)
+ return 0;
+
+ /* task_lock held in oom_badness */
+ smaps_walk.mm = mm;
+
+ if (!down_read_trylock(&mm->mmap_sem)) {
+ pr_warn("Skipping task:%s\n", p->comm);
+ return 0;
+ }
+
+ vma = mm->mmap;
+ if (!vma) {
+ up_read(&mm->mmap_sem);
+ return 0;
+ }
+
+ while (vma) {
+ memset(&mss, 0, sizeof(struct mem_size_stats));
+ walk_page_vma(vma, &smaps_walk);
+ pss += (unsigned long) (mss.pss >> (12 + PSS_SHIFT)); /*PSS in PAGE */
+
+ /* Check next vma in list */
+ vma = vma->vm_next;
+ }
+ up_read(&mm->mmap_sem);
+ return pss;
+}
+
#define PAGEMAP_WALK_SIZE (PMD_SIZE)
#define PAGEMAP_WALK_MASK (PMD_MASK)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 47a9392..b6bb521 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1423,6 +1423,15 @@ static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
*maxrss = hiwater_rss;
}
+#ifdef CONFIG_PROC_PAGE_MONITOR
+long get_mm_pss(struct task_struct *p, struct mm_struct *mm);
+#else
+static inline long get_mm_pss(struct task_struct *p, struct mm_struct *mm)
+{
+ return 0;
+}
+#endif
+
#if defined(SPLIT_RSS_COUNTING)
void sync_mm_rss(struct mm_struct *mm);
#else
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 642f38c..537eb4c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -151,6 +151,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
{
long points;
long adj;
+ long pss = 0;
if (oom_unkillable_task(p, memcg, nodemask))
return 0;
@@ -167,9 +168,13 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
/*
* The baseline for the badness score is the proportion of RAM that each
- * task's rss, pagetable and swap space use.
+ * task's pss, pagetable and swap space use.
*/
- points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
+ pss = get_mm_pss(p, p->mm);
+ if (pss == 0) /* make pss equals to rss, pseudo-pss */
+ pss = get_mm_rss(p->mm);
+
+ points = pss + get_mm_counter(p->mm, MM_SWAPENTS) +
atomic_long_read(&p->mm->nr_ptes) + mm_nr_pmds(p->mm);
task_unlock(p);
--
1.7.1
--
BRs
Yogesh Gaur.????{.n?+???????+%?????ݶ??w??{.n?+????{??G?????{ay? ʇڙ?,j??f???h?????????z_??(?階?ݢj"???m??????G????????????&???~???iO???z??v?^?m???? ????????I?
2015-05-08 16:01 GMT+08:00 Yogesh Narayan Gaur <[email protected]>:
> EP-2DAD0AFA905A4ACB804C4F82A001242F
>
> ------- Original Message -------
> Sender : yalin wang<[email protected]>
> Date : May 08, 2015 13:17 (GMT+05:30)
> Title : Re: [EDT] oom_killer: find bulkiest task based on pss value
>
> 2015-05-08 13:29 GMT+08:00 Yogesh Narayan Gaur :
>>>
>>> EP-2DAD0AFA905A4ACB804C4F82A001242F
>>> Hi Andrew,
>>>
>>> Presently in oom_kill.c we calculate badness score of the victim task as per the present RSS counter value of the task.
>>> RSS counter value for any task is usually '[Private (Dirty/Clean)] + [Shared (Dirty/Clean)]' of the task.
>>> We have encountered a situation where values for Private fields are less but value for Shared fields are more and hence make total RSS counter value large. Later on oom situation killing task with highest RSS value but as Private field values are not large hence memory gain after killing this process is not as per the expectation.
>>>
>>> For e.g. take below use-case scenario, in which 3 process are running in system.
>>> All these process done mmap for file exist in present directory and then copying data from this file to local allocated pointers in while(1) loop with some sleep. Out of 3 process, 2 process has mmaped file with MAP_SHARED setting and one has mapped file with MAP_PRIVATE setting.
>>> I have all 3 processes in background and checks RSS/PSS value from user space utility (utility over cat /proc/pid/smaps)
>>> Before OOM, below is the consumed memory status for these 3 process (all processes run with oom_score_adj = 0)
>>> ====================================================
>>> Comm : 1prg, Pid : 213 (values in kB)
>>> Rss Shared Private Pss
>>> Process : 375764 194596 181168 278460
>>> ====================================================
>>> Comm : 3prg, Pid : 217 (values in kB)
>>> Rss Shared Private Pss
>>> Process : 305760 32 305728 305738
>>> ====================================================
>>> Comm : 2prg, Pid : 218 (values in kB)
>>> Rss Shared Private Pss
>>> Process : 389980 194596 195384 292676
>>> ====================================================
>>>
>>> Thus as per present code design, first it would select process [2prg : 218] as bulkiest process as its RSS value is highest to kill. But if we kill this process then only ~195MB would be free as compare to expected ~389MB.
>>> Thus identifying the task based on RSS value is not accurate design and killing that identified process didn’t release expected memory back to system.
>>>
>>> We need to calculate victim task based on PSS instead of RSS as PSS value calculates as
>>> PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
>>> For above use-case scenario also, it can be checked that process [3prg : 217] is having largest PSS value and by killing this process we can gain maximum memory (~305MB) as compare to killing process identified based on RSS value.
>>>
>>> --
>>> Regards,
>>> Yogesh Gaur.
>
>>
>>Great,
>>
>> in fact, i also encounter this scenario,
>> I use USS (page map counter == 1) pages
>> to decide which process should be killed,
>> seems have the same result as you use PSS,
>> but PSS is better , it also consider shared pages,
>> in case some process have large shared pages mapping
>> but little Private page mapping
>>
>> BRs,
>> Yalin
>
> I have made patch which identifies bulkiest task on basis of PSS value. Please check below patch.
> This patch is correcting the way victim task gets identified in oom condition.
>
> ==================
>
> From 1c3d7f552f696bdbc0126c8e23beabedbd80e423 Mon Sep 17 00:00:00 2001
> From: Yogesh Gaur <[email protected]>
> Date: Thu, 7 May 2015 01:52:13 +0530
> Subject: [PATCH] oom: find victim task based on pss
>
> This patch is identifying bulkiest task to kill by OOM on the basis of PSS value
> instead of present RSS values.
> There can be scenario where task with highest RSS counter is consuming lot of shared
> memory and killing that task didn't release expected amount of memory to system.
> PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
> RSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean)]
> Thus, using PSS value instead of RSS value as PSS value closely matches with actual
> memory usage by the task.
> This patch is using smaps_pte_range() interface defined in CONFIG_PROC_PAGE_MONITOR.
> For case when CONFIG_PROC_PAGE_MONITOR disabled, this simply returns RSS value count.
>
> Signed-off-by: Yogesh Gaur <[email protected]>
> Signed-off-by: Amit Arora <[email protected]>
> Reviewed-by: Ajeet Yadav <[email protected]>
> ---
> fs/proc/task_mmu.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/mm.h | 9 +++++++++
> mm/oom_kill.c | 9 +++++++--
> 3 files changed, 63 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 956b75d..dd962ff 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -964,6 +964,53 @@ struct pagemapread {
> bool v2;
> };
>
> +/**
> + * get_mm_pss - function to determine PSS count of pages being used for proc p
> + * PSS value=[Private(Dirty/Clean)] + [Shared(Dirty/Clean)/no. of shared task]
> + * @p: task struct of which task we should calculate
> + * @mm: mm struct of the task.
> + *
> + * This function needs to be called under task_lock for calling task 'p'.
> + */
> +long get_mm_pss(struct task_struct *p, struct mm_struct *mm)
> +{
> + long pss = 0;
> + struct vm_area_struct *vma = NULL;
> + struct mem_size_stats mss;
> + struct mm_walk smaps_walk = {
> + .pmd_entry = smaps_pte_range,
> + .private = &mss,
> + };
> +
> + if (mm == NULL)
> + return 0;
> +
> + /* task_lock held in oom_badness */
> + smaps_walk.mm = mm;
> +
> + if (!down_read_trylock(&mm->mmap_sem)) {
> + pr_warn("Skipping task:%s\n", p->comm);
> + return 0;
> + }
> +
> + vma = mm->mmap;
> + if (!vma) {
> + up_read(&mm->mmap_sem);
> + return 0;
> + }
> +
> + while (vma) {
> + memset(&mss, 0, sizeof(struct mem_size_stats));
> + walk_page_vma(vma, &smaps_walk);
> + pss += (unsigned long) (mss.pss >> (12 + PSS_SHIFT)); /*PSS in PAGE */
> +
> + /* Check next vma in list */
> + vma = vma->vm_next;
> + }
> + up_read(&mm->mmap_sem);
> + return pss;
> +}
> +
> #define PAGEMAP_WALK_SIZE (PMD_SIZE)
> #define PAGEMAP_WALK_MASK (PMD_MASK)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 47a9392..b6bb521 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1423,6 +1423,15 @@ static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
> *maxrss = hiwater_rss;
> }
>
> +#ifdef CONFIG_PROC_PAGE_MONITOR
> +long get_mm_pss(struct task_struct *p, struct mm_struct *mm);
> +#else
> +static inline long get_mm_pss(struct task_struct *p, struct mm_struct *mm)
> +{
> + return 0;
> +}
> +#endif
> +
> #if defined(SPLIT_RSS_COUNTING)
> void sync_mm_rss(struct mm_struct *mm);
> #else
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 642f38c..537eb4c 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -151,6 +151,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
> {
> long points;
> long adj;
> + long pss = 0;
>
> if (oom_unkillable_task(p, memcg, nodemask))
> return 0;
> @@ -167,9 +168,13 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
>
> /*
> * The baseline for the badness score is the proportion of RAM that each
> - * task's rss, pagetable and swap space use.
> + * task's pss, pagetable and swap space use.
> */
> - points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
> + pss = get_mm_pss(p, p->mm);
> + if (pss == 0) /* make pss equals to rss, pseudo-pss */
> + pss = get_mm_rss(p->mm);
> +
> + points = pss + get_mm_counter(p->mm, MM_SWAPENTS) +
> atomic_long_read(&p->mm->nr_ptes) + mm_nr_pmds(p->mm);
> task_unlock(p);
>
> --
> 1.7.1
>
>
> --
> BRs
> Yogesh Gaur.
The logic seems ok ,
but i feel the code footprint is too large than the original method,
maybe have some performance problems,
i have add linux-mm mail list for mm expert to review .
BRs
Yalin.
On Fri, 8 May 2015, Yogesh Narayan Gaur wrote:
> Presently in oom_kill.c we calculate badness score of the victim task as per the present RSS counter value of the task.
> RSS counter value for any task is usually '[Private (Dirty/Clean)] + [Shared (Dirty/Clean)]' of the task.
> We have encountered a situation where values for Private fields are less but value for Shared fields are more and hence make total RSS counter value large. Later on oom situation killing task with highest RSS value but as Private field values are not large hence memory gain after killing this process is not as per the expectation.
>
> For e.g. take below use-case scenario, in which 3 process are running in system.
> All these process done mmap for file exist in present directory and then copying data from this file to local allocated pointers in while(1) loop with some sleep. Out of 3 process, 2 process has mmaped file with MAP_SHARED setting and one has mapped file with MAP_PRIVATE setting.
> I have all 3 processes in background and checks RSS/PSS value from user space utility (utility over cat /proc/pid/smaps)
> Before OOM, below is the consumed memory status for these 3 process (all processes run with oom_score_adj = 0)
> ====================================================
> Comm : 1prg, Pid : 213 (values in kB)
> Rss Shared Private Pss
> Process : 375764 194596 181168 278460
> ====================================================
> Comm : 3prg, Pid : 217 (values in kB)
> Rss Shared Private Pss
> Process : 305760 32 305728 305738
> ====================================================
> Comm : 2prg, Pid : 218 (values in kB)
> Rss Shared Private Pss
> Process : 389980 194596 195384 292676
> ====================================================
>
> Thus as per present code design, first it would select process [2prg : 218] as bulkiest process as its RSS value is highest to kill. But if we kill this process then only ~195MB would be free as compare to expected ~389MB.
> Thus identifying the task based on RSS value is not accurate design and killing that identified process didn?t release expected memory back to system.
>
> We need to calculate victim task based on PSS instead of RSS as PSS value calculates as
> PSS value = [Private (Dirty/Clean)] + [Shared (Dirty/Clean) / no. of shared task]
> For above use-case scenario also, it can be checked that process [3prg : 217] is having largest PSS value and by killing this process we can gain maximum memory (~305MB) as compare to killing process identified based on RSS value.
>
The oom killer doesn't expect to necessarily be able to free all memory
that is represented by the rss of a process. In fact, after it selects a
process it will happily kill a child process in favor of its parent if
they don't share the same memory.
There're a few problems with using pss and the proposed patch that
follows:
- it's less predictable since it depends on the number of times the
memory is mapped, which may change during the process's lifetime,
- it requires mm->mmap_sem to do, which is not possible to do because
it may be held and thus reverting back to rss in situations where
the trylock fails makes it even less predictable and reliable, and
- all users who currently tune /proc/pid/oom_score_adj or
/proc/pid/oom_adj are doing so based on the current heuristic, which
is rss; if we switched to pss and all a process's memory is shared
then their oom_score_adj or oom_adj is now severely broken (and as a
result of the first problem above, defining oom_score_adj is near
impossible).
We don't have the expectation of freeing the entire rss, the best we can
do is use a heuristic which is reliable, consistent, and cheap to check.
We can then ask users who desire a process to have a different oom kill
priority to use oom_score_adj and they may do so in a reliable way without
having the fallback behavior that your trylock does.