Hello,
在 2023/8/4 19:29, Michal Hocko 写道:
On Fri 04-08-23 17:38:03, Chuyi Zhou wrote:
This patch adds a new hook bpf_select_task in oom_evaluate_task. It
takes oc and current iterating task as parameters and returns a result
indicating which one is selected by bpf program.
Although bpf_select_task is used to bypass the default method, there are
some existing rules should be obeyed. Specifically, we skip these
"unkillable" tasks(e.g., kthread, MMF_OOM_SKIP, in_vfork()).So we do not
consider tasks with lowest score returned by oom_badness except it was
caused by OOM_SCORE_ADJ_MIN.
Is this really necessary? I do get why we need to preserve
OOM_SCORE_ADJ_* semantic for in-kernel oom selection logic but why
should an arbitrary oom policy care. Look at it from an arbitrary user
space based policy. It just picks a task or memcg and kills taks by
sending SIG_KILL (or maybe SIG_TERM first) signal. oom_score constrains
will not prevent anybody from doing that.
Sorry, some of my expressions may have misled you.
I do agree bpf interface should bypass the current OOM_SCORE_ADJ_*
logic. What I meant to say is that bpf can select a task even it was
setted OOM_SCORE_ADJ_MIN.
tsk_is_oom_victim (and MMF_OOM_SKIP) is a slightly different case but
not too much. The primary motivation is to prevent new oom victims
while there is one already being killed. This is a reasonable heuristic
especially with the async oom reclaim (oom_reaper). It also reduces
amount of oom emergency memory reserves to some degree but since those
are not absolute this is no longer the primary motivation. _But_ I can
imagine that some policies might be much more aggresive and allow to
select new victims if preexisting are not being killed in time.
oom_unkillable_task is a general sanity check so it should remain in
place.
I am not really sure about oom_task_origin. That is just a very weird
case and I guess it wouldn't hurt to keep it in generic path.
All that being said I think we want something like the following (very
pseudo-code). I have no idea what is the proper way how to define BPF
hooks though so a help from BPF maintainers would be more then handy
---
diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 00982b133dc1..9f1743ee2b28 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -190,10 +190,6 @@ static inline bool trigger_all_cpu_backtrace(void)
{
return false;
}
-static inline bool trigger_allbutself_cpu_backtrace(void)
-{
- return false;
-}
static inline bool trigger_cpumask_backtrace(struct cpumask *mask)
{
return false;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 612b5597d3af..c9e04be52700 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -317,6 +317,22 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
if (!is_memcg_oom(oc) && !oom_cpuset_eligible(task, oc))
goto next;
+ /*
+ * If task is allocating a lot of memory and has been marked to be
+ * killed first if it triggers an oom, then select it.
+ */
+ if (oom_task_origin(task)) {
+ points = LONG_MAX;
+ goto select;
+ }
+
+ switch (bpf_oom_evaluate_task(task, oc, &points)) {
+ case -EOPNOTSUPP: break; /* No BPF policy */
+ case -EBUSY: goto abort; /* abort search process */
+ case 0: goto next; /* ignore process */
+ default: goto select; /* note the task */
+ }
Why we need to change the *points* value if we do not care about
oom_badness ? Is it used to record some state? If so, we could record it
through bpf map.
+
/*
* This task already has access to memory reserves and is being killed.
* Don't allow any other task to have access to the reserves unless
@@ -329,15 +345,6 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
goto abort;
}
- /*
- * If task is allocating a lot of memory and has been marked to be
- * killed first if it triggers an oom, then select it.
- */
- if (oom_task_origin(task)) {
- points = LONG_MAX;
- goto select;
- }
-
points = oom_badness(task, oc->totalpages);
if (points == LONG_MIN || points < oc->chosen_points)
goto next;
Thanks for your advice, I'm very glad to follow your suggestions for the
next version of development.
