kprobes: kretprobe events missing on 2-core KVM guest

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@bytedance.com/Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
Reviewed-by: Solar Designer <solar@openwall.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

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)