This is a note to let you know that I've just added the patch titled
kprobes: kretprobe events missing on 2-core KVM guest
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kprobes-kretprobe-events-missing-on-2-core-kvm-guest.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 0ef03bcbb1b8d23660a75bd7f2012a61dab1654a
Author: wuqiang <wuqiang.matt@xxxxxxxxxxxxx>
Date: Thu Nov 10 16:15:02 2022 +0800
kprobes: kretprobe events missing on 2-core KVM guest
[ Upstream commit 3b7ddab8a19aefc768f345fd3782af35b4a68d9b ]
Default value of maxactive is set as num_possible_cpus() for nonpreemptable
systems. For a 2-core system, only 2 kretprobe instances would be allocated
in default, then these 2 instances for execve kretprobe are very likely to
be used up with a pipelined command.
Here's the testcase: a shell script was added to crontab, and the content
of the script is:
#!/bin/sh
do_something_magic `tr -dc a-z < /dev/urandom | head -c 10`
cron will trigger a series of program executions (4 times every hour). Then
events loss would be noticed normally after 3-4 hours of testings.
The issue is caused by a burst of series of execve requests. The best number
of kretprobe instances could be different case by case, and should be user's
duty to determine, but num_possible_cpus() as the default value is inadequate
especially for systems with small number of cpus.
This patch enables the logic for preemption as default, thus increases the
minimum of maxactive to 10 for nonpreemptable systems.
Link: https://lore.kernel.org/all/20221110081502.492289-1-wuqiang.matt@xxxxxxxxxxxxx/
Signed-off-by: wuqiang <wuqiang.matt@xxxxxxxxxxxxx>
Reviewed-by: Solar Designer <solar@xxxxxxxxxxxx>
Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/Documentation/trace/kprobes.rst b/Documentation/trace/kprobes.rst
index 48cf778a2468..fc7ce76eab65 100644
--- a/Documentation/trace/kprobes.rst
+++ b/Documentation/trace/kprobes.rst
@@ -131,8 +131,7 @@ For example, if the function is non-recursive and is called with a
spinlock held, maxactive = 1 should be enough. If the function is
non-recursive and can never relinquish the CPU (e.g., via a semaphore
or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
-set to a default value. If CONFIG_PREEMPT is enabled, the default
-is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
+set to a default value: max(10, 2*NR_CPUS).
It's not a disaster if you set maxactive too low; you'll just miss
some probes. In the kretprobe struct, the nmissed field is set to
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index a35074f0daa1..1c18ecf9f98b 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -2213,13 +2213,9 @@ int register_kretprobe(struct kretprobe *rp)
rp->kp.post_handler = NULL;
/* Pre-allocate memory for max kretprobe instances */
- if (rp->maxactive <= 0) {
-#ifdef CONFIG_PREEMPTION
+ if (rp->maxactive <= 0)
rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
-#else
- rp->maxactive = num_possible_cpus();
-#endif
- }
+
#ifdef CONFIG_KRETPROBE_ON_RETHOOK
rp->rh = rethook_alloc((void *)rp, kretprobe_rethook_handler);
if (!rp->rh)
