diff mbox series

kretprobe events missing on 2-core KVM guest

Message ID 20221025100117.18667-1-wuqiang.matt@bytedance.com
State New
Headers
Series kretprobe events missing on 2-core KVM guest |

Commit Message

wuqiang.matt Oct. 25, 2022, 10:01 a.m. UTC 
  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.

This patch increases the minimum of maxactive to 10.

Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
---
 kernel/kprobes.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Masami Hiramatsu (Google) Oct. 25, 2022, 3:33 p.m. UTC | #1 
On Tue, 25 Oct 2022 18:01:17 +0800
wuqiang <wuqiang.matt@bytedance.com> wrote:

> 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.
> 
> This patch increases the minimum of maxactive to 10.
> 

This looks reasonable to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you!

> Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
> ---
>  kernel/kprobes.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 3220b0a2fb4a..b781dee3f552 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -2211,7 +2211,7 @@ int register_kretprobe(struct kretprobe *rp)
>  #ifdef CONFIG_PREEMPTION
>  		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
>  #else
> -		rp->maxactive = num_possible_cpus();
> +		rp->maxactive = max_t(unsigned int, 10, num_possible_cpus());
>  #endif
>  	}
>  #ifdef CONFIG_KRETPROBE_ON_RETHOOK
> -- 
> 2.34.1
>
Solar Designer Nov. 7, 2022, 1:36 p.m. UTC | #2 
On Wed, Oct 26, 2022 at 12:33:15AM +0900, Masami Hiramatsu wrote:
> On Tue, 25 Oct 2022 18:01:17 +0800
> wuqiang <wuqiang.matt@bytedance.com> wrote:
> 
> > 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.
> > 
> > This patch increases the minimum of maxactive to 10.
> 
> This looks reasonable to me.
> 
> Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Reasonable yes, but:

Is 10 enough?  How exactly do those instances get used up without
preemption?  Perhaps because execve can sleep?  If so, perhaps we should
use the same logic without preemption that we do with preemption?  So
maybe just make this line unconditional? -

		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());

Also, the behavior was documented in Documentation/trace/kprobes.rst, so
perhaps that file should be updated at the same time with the code.

> > Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
> > ---
> >  kernel/kprobes.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 3220b0a2fb4a..b781dee3f552 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -2211,7 +2211,7 @@ int register_kretprobe(struct kretprobe *rp)
> >  #ifdef CONFIG_PREEMPTION
> >  		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
> >  #else
> > -		rp->maxactive = num_possible_cpus();
> > +		rp->maxactive = max_t(unsigned int, 10, num_possible_cpus());
> >  #endif
> >  	}
> >  #ifdef CONFIG_KRETPROBE_ON_RETHOOK
> > -- 
> > 2.34.1
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

Alexander
wuqiang.matt Nov. 8, 2022, 2:50 a.m. UTC | #3 
On 2022/11/7 21:36, Solar Designer wrote:
> On Wed, Oct 26, 2022 at 12:33:15AM +0900, Masami Hiramatsu wrote:
>> On Tue, 25 Oct 2022 18:01:17 +0800
>> wuqiang <wuqiang.matt@bytedance.com> wrote:
>>
>>> 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.
>>>
>>> This patch increases the minimum of maxactive to 10.
>>
>> This looks reasonable to me.
>>
>> Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Reasonable yes, but:
> 
> Is 10 enough?  How exactly do those instances get used up without
> preemption?  Perhaps because execve can sleep?  If so, perhaps we should
> use the same logic without preemption that we do with preemption?  So
> maybe just make this line unconditional? -
> 
> 		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());

I agree to make it unconditional, though it could cost a bit more memory.

Here's my testcase: a shell script was added to crontab, and the content of
the script is:

   #!/bin/sh
   do_something_with_magic `tr -dc a-z < /dev/urandom | head -c 10`

cron will trigger a series of program executions (4 times every hour). Then
we noticed events loss after 3-4 hours of testings.

The issue is caused by a burst of series of execve requests. The best number
of instances could be different case by case, and should be the user's duty
to decide, but num_possible_cpus() as a default value is inadequate. For my
testcase, 8 is enough as verified, and 10 is chosen to keep it identical.

The handling of execve syscall can be suspended or voluntarily yield up cpu
due to i/o or memory resources and then a new execve gets its time slice to
start. It could be worse for scenarios of resource throttling or routines
that are heavier than execve (though rare as I think).

> Also, the behavior was documented in Documentation/trace/kprobes.rst, so
> perhaps that file should be updated at the same time with the code.

Right, will update in next version.

>>> Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
>>> ---
>>>   kernel/kprobes.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>>> index 3220b0a2fb4a..b781dee3f552 100644
>>> --- a/kernel/kprobes.c
>>> +++ b/kernel/kprobes.c
>>> @@ -2211,7 +2211,7 @@ int register_kretprobe(struct kretprobe *rp)
>>>   #ifdef CONFIG_PREEMPTION
>>>   		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
>>>   #else
>>> -		rp->maxactive = num_possible_cpus();
>>> +		rp->maxactive = max_t(unsigned int, 10, num_possible_cpus());
>>>   #endif
>>>   	}
>>>   #ifdef CONFIG_KRETPROBE_ON_RETHOOK
>>> -- 
>>> 2.34.1
>>>
>>
>>
>> -- 
>> Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Thanks,
> 
> Alexander

Best regards,
wuqiang
diff mbox series

Patch

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 3220b0a2fb4a..b781dee3f552 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -2211,7 +2211,7 @@  int register_kretprobe(struct kretprobe *rp)
 #ifdef CONFIG_PREEMPTION
 		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
 #else
-		rp->maxactive = num_possible_cpus();
+		rp->maxactive = max_t(unsigned int, 10, num_possible_cpus());
 #endif
 	}
 #ifdef CONFIG_KRETPROBE_ON_RETHOOK