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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>
This commit is contained in:
wuqiang 2022-11-10 16:15:02 +08:00 committed by Masami Hiramatsu (Google)
parent 0c76ef3f26
commit 3b7ddab8a1
2 changed files with 3 additions and 8 deletions
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3
Documentation/trace/kprobes.rst
View File

@ -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 spinlock held, maxactive = 1 should be enough. If the function is
non-recursive and can never relinquish the CPU (e.g., via a semaphore 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 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
set to a default value. If CONFIG_PREEMPT is enabled, the default set to a default value: max(10, 2*NR_CPUS).
is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
It's not a disaster if you set maxactive too low; you'll just miss 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 some probes. In the kretprobe struct, the nmissed field is set to

8
kernel/kprobes.c
View File

@ -2213,13 +2213,9 @@ int register_kretprobe(struct kretprobe *rp)
rp->kp.post_handler = NULL; rp->kp.post_handler = NULL;
/* Pre-allocate memory for max kretprobe instances */ /* Pre-allocate memory for max kretprobe instances */
if (rp->maxactive <= 0) { if (rp->maxactive <= 0)
#ifdef CONFIG_PREEMPTION
rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus()); rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
#else
rp->maxactive = num_possible_cpus();
#endif
}
#ifdef CONFIG_KRETPROBE_ON_RETHOOK #ifdef CONFIG_KRETPROBE_ON_RETHOOK
rp->rh = rethook_alloc((void *)rp, kretprobe_rethook_handler); rp->rh = rethook_alloc((void *)rp, kretprobe_rethook_handler);
if (!rp->rh) if (!rp->rh)