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From: Jeff Hostetler <git@jeffhostetler.com>
To: Josh Steadmon <steadmon@google.com>,
	Junio C Hamano <gitster@pobox.com>,
	git@vger.kernel.org
Subject: Re: [PATCH] run-command: don't spam trace2_child_exit()
Date: Tue, 10 May 2022 16:37:33 -0400	[thread overview]
Message-ID: <ef011c3d-dd4b-f982-aa66-9167bf3e6c91@jeffhostetler.com> (raw)
In-Reply-To: <YnQsTckTqRaKLABM@google.com>



On 5/5/22 3:58 PM, Josh Steadmon wrote:
> On 2022.05.03 10:59, Jeff Hostetler wrote:
>>
>>
>> On 4/28/22 5:46 PM, Junio C Hamano wrote:
>>> Josh Steadmon <steadmon@google.com> writes:
>>>
>>>> In rare cases, wait_or_whine() cannot determine a child process's exit
>>>> status (and will return -1 in this case). This can cause Git to issue
>>>> trace2 child_exit events despite the fact that the child is still
>>>> running.
>>
>> I'm curious what is causing the spurious return values.
>> Could you instrument wait_or_whine() and see which of the
>> if/else arms are causing the -1 to be returned?
>>
>> That routine is rather complicated and looks like it has 3
>> different ways that a -1 could be returned.
> 
> Unfortunately we haven't been able to reproduce the issue, so
> instrumentation won't clarify the cause unless we randomly hit another
> instance.
> 
> 
>>>
>>> Rather, we do not even know if the child is still running when it
>>> happens, right?  It is curious what "rare cases" makes the symptom
>>> appear.  Do we know?
>>>
>>> The patch looks OK from the "we do not know the child exited in this
>>> case, so we shouldn't be reporting the child exit" point of view, of
>>> course.  Having one event that started a child in the log and then
>>> having millions of events that reports the exit of the (same) child
>>> is way too broken.  With this change, we remove these phoney exit
>>> events from the log.
>>>
>>> Do we know, for such a child process that caused these millions
>>> phoney exit events, we got a real exit event at the end?  Otherwise,
>>> we'd still have a similar problem in the opposite direction, i.e. a
>>> child has a start event recorded, many exit event discarded but the
>>> log lacks the true exit event for the child, implying that the child
>>> is still running because we failed to log its exit?
>>>
>>>>    int finish_command_in_signal(struct child_process *cmd)
>>>>    {
>>>>    	int ret = wait_or_whine(cmd->pid, cmd->args.v[0], 1);
>>>> -	trace2_child_exit(cmd, ret);
>>>> +	if (ret != -1)
>>>> +		trace2_child_exit(cmd, ret);
>>>>    	return ret;
>>>>    }
>>
>> Since this is only called from pager.c and is used to setup the
>> pager, I have to wonder if you're only getting these spurious events
>> for the pager process or for any of the other random child processes.
>>
>> And whether they are received while the pager is alive and working
>> properly, or when you're trying to quit the pager or when the pager
>> is trying to signal eof.
> 
> It was for a pager run by `git diff`. The only other child process in
> this instance was `git status`. I believe the events were generated
> while the user was not interacting with the process; IIRC the pager was
> left running in a terminal overnight and they discovered the next
> morning that their disk was filled up by trace logs.
> 

This worries me that you've found a race or infinite loop here
and the only symptom is the child exit messages.  Suppressing
them solves the disk-fill problem, but might mean that there's
a lurking infinite loop or something.

Can you (privately) send me the part of the log before the
infinite loop?  And maybe the parent and child processes too?
I'd kinda like to understand what's happening here.

Jeff


  reply	other threads:[~2022-05-10 20:37 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-28 20:58 Josh Steadmon
2022-04-28 21:46 ` Junio C Hamano
2022-05-03 14:59   ` Jeff Hostetler
2022-05-05 19:58     ` Josh Steadmon
2022-05-10 20:37       ` Jeff Hostetler [this message]
2022-06-07 18:45         ` Josh Steadmon
2022-05-05 19:44   ` Josh Steadmon
2022-06-07 18:21 ` [PATCH v2] " Josh Steadmon
2022-06-07 22:09   ` Ævar Arnfjörð Bjarmason
2022-06-10 15:31   ` Jeff Hostetler

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