Re: [PATCH] show message when exceeded rlimit of pending signals

[Naohiro Ooiwa <nooiwa@miraclelinux.com>]

Hi Ingo, Roland,

Now, I received a nice comment from OGAWA-san.
How is this impriment like a print_faital_signal().

I think it's very nice.

Thank you
Naohiro Ooiwa.


Naohiro Ooiwa wrote:
> Hi Ingo
> 
> Thank you so much for early quick reply.
> and I'm happy you agree with my proposal.
> 
>> Regarding the patch, i've got a few (very) small suggestions.
> 
> Thank you for pointing out.
> Please wait a moment. I will resend a patch.
> 
> Of course, I will plan to use print_ratelimit().
> Actually, I received with same opinion from OGAWA-san.
> 
> 
> Thank you
> Naohiro Ooiwa.
> 
> 
> Ingo Molnar wrote:
>> * Naohiro Ooiwa <nooiwa@miraclelinux.com> wrote:
>>
>>> Hi Andrew,
>>>
>>> I was glad to talk to you in Japan Linux Symposium.
>>> I'm writing about it.
>>>
>>>
>>> I'm working to support kernel.
>>> Recently, I got a inquiry about unexpected system behavior.
>>> I analyzed application of our customer includeing kernel.
>>>
>>> Eventually, there was no bug in application or kernel.
>>> I found the cause was the limit of pending signals.
>>> I ran following command. and system behaved expectedly.
>>>    # ulimit -i unlimited
>>>
>>> When system behaved unexpectedly, the timer_create() in application
>>> had returned -EAGAIN value.
>>> But we can't imagine the -EAGAIN means that it exceeded limit of
>>> pending signals at all.
>>>
>>> Then I thought kernel should at least show some message about it.
>>> And I tried to create a patch.
>>>
>>> I'm sure that system engineeres will not have to have the same 
>>> experience as I did.
>>> How do you think about this idea ?
>>>
>>> Thank you
>>> Naohiro Ooiwa.
>>>
>>> Signed-off-by: Naohiro Ooiwa <nooiwa@miraclelinux.com>
>>> ---
>>>  kernel/signal.c |   13 +++++++++++++
>>>  1 files changed, 13 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/kernel/signal.c b/kernel/signal.c
>>> index 6705320..0bc4934 100644
>>> --- a/kernel/signal.c
>>> +++ b/kernel/signal.c
>>> @@ -188,6 +188,9 @@ int next_signal(struct sigpending *pending, 
>>> sigset_t *mask)
>>>      return sig;
>>>  }
>>>
>>> +#define MAX_RLIMIT_CAUTION 5
>>> +static int rlimit_caution_count = 0;
>>> +
>>>  /*
>>>   * allocate a new signal queue record
>>>   * - this may be called without locks if and only if t == current, 
>>> otherwise an
>>> @@ -211,6 +214,16 @@ static struct sigqueue *__sigqueue_alloc(struct 
>>> task_struct *t, gfp_t flags,
>>>          atomic_read(&user->sigpending) <=
>>>              t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
>>>          q = kmem_cache_alloc(sigqueue_cachep, flags);
>>> +    else {
>>> +        if (rlimit_caution_count <= MAX_RLIMIT_CAUTION ){
>>> +            printk(KERN_WARNING "reached the limit of pending 
>>> signalis on pid %d\n", current->pid);
>>> +            /* Last time, show the advice */
>>> +            if (rlimit_caution_count == MAX_RLIMIT_CAUTION)
>>> +                printk(KERN_WARNING "If unexpected your system 
>>> behavior, you can try ulimit -i unlimited\n");
>>> +            rlimit_caution_count++;
>>> +        }
>>> +    }
>>> +
>>>      if (unlikely(q == NULL)) {
>>>          atomic_dec(&user->sigpending);
>>>          free_uid(user);
>>
>> This new warning looks quite useful, i've seen several apps get into 
>> trouble silently due to that, again and again.
>>
>> The memory overhead of the signal queue was a problem 15 years ago ... 
>> not so much today and people (and apps) dont expect to get in trouble 
>> here. So the limit and its defaults are somewhat arcane, and the 
>> behavior is catastrophic and hard to debug (because it's a dynamic 
>> failure).
>>
>> Regarding the patch, i've got a few (very) small suggestions.
>>
>> Firstly, please update the if / else sequence from:
>>
>>     if (...)
>>         ...
>>     else {
>>         ...
>>     }
>>
>> to:
>>
>>     if (...) {
>>         ...
>>     } else {
>>         ...
>>     }
>>
>> as we strive for curly brace symmetries.
>>
>> also, a small typo: s/signalis/signals
>>
>> Plus, instead of using a pre-cooked global limit print_ratelimit() 
>> could be used as well. That makes it useful for long-lived systems 
>> that run into this limit occasionally. We wont spam the log - nor will 
>> we lose (potentially essential) messages in the process.
>>
>> Thanks,
>>
>>     Ingo
> 
> 
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