linux-kernel - Re: [PATCH v3] kernel/module: Reschedule while waiting for modules to finish loading

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Date:   Thu, 2 May 2019 13:46:16 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        David Arcari <darcari@...hat.com>
Subject: Re: [PATCH v3] kernel/module: Reschedule while waiting for modules to
 finish loading



On 5/2/19 8:41 AM, Prarit Bhargava wrote:
> 
> 
> On 5/2/19 5:48 AM, Jessica Yu wrote:
>> +++ Prarit Bhargava [01/05/19 17:26 -0400]:
>>>
>>>
>>> On 4/30/19 6:22 PM, Prarit Bhargava wrote:
>>>> On a s390 z14 LAR with 2 cpus about stalls about 3% of the time while
>>>> loading the s390_trng.ko module.
>>>>
>>>> Add a reschedule point to the loop that waits for modules to complete
>>>> loading.
>>>>
>>>> v3: cleanup Fixes line.
>>>
>>> Jessica, even with this additional patch there appears to be some other issues
>>> in the module code that are causing significant delays in boot up on large
>>> systems.
>>
>> Is this limited to only s390? Or are you seeing this on other arches
>> as well? And is it limited to specific modules (like s390_trng)?
> 
> Other arches.  We're seeing a hang on a new 192 CPU x86_64 box & the
> acpi_cpufreq driver.  The system is MUCH faster than any other x86_64 box I've
> seen and that's likely why I'm seeing a problem.
> 
>>
>>> FWIW, the logic in the original patch is correct.  It's just that there's, as
>>> Heiko discovered, some poor scheduling, etc., that is impacting the module
>>> loading code after these changes.
>>
>> I am really curious to see what these performance regressions look
>> like :/ Please update us when you find out more.
>>
> 
> I sent Heiko a private v4 RFC last night with this patch (sorry for the
> cut-and-paste)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 1c429d8d2d74..a4ef8628f26f 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3568,12 +3568,12 @@ static int add_unformed_module(struct module *mod)
> 	mutex_lock(&module_mutex);
> 	old = find_module_all(mod->name, strlen(mod->name), true);
> 	if (old != NULL) {
> -		if (old->state == MODULE_STATE_COMING
> -		    || old->state == MODULE_STATE_UNFORMED) {
> +		if (old->state != MODULE_STATE_LIVE) {
> 			/* Wait in case it fails to load. */
> 			mutex_unlock(&module_mutex);
> -			err = wait_event_interruptible(module_wq,
> -					       finished_loading(mod->name));
> +			err = wait_event_interruptible_timeout(module_wq,
> +					       finished_loading(mod->name),
> +					       HZ / 10000);
> 			if (err)
> 				goto out_unlocked;
> 			goto again;
> 
> The original module dependency race issue is fixed simply by changing the
> conditional to checking !MODULE_STATE_LIVE.  This, unfortunately, exposed some
> other problems within the code.
> 
> The module_wq is only run when a module fails to load.  It's possible that
> the time between the module's failed init() call and running module_wq
> (kernel/module.c:3455) takes a while.  Any thread entering the
> add_unformed_module() code while the old module is unloading is put to sleep
> waiting for the module_wq to execute.
> 
> On the 192 thread box I have noticed that the acpi_cpufreq module attempts
> to load 392 times (that is not a typo and I am going to try to figure that
> problem out after this one).  This means 191 cpus are put to sleep, and one
> cpu is executing the acpi_cpufreq module unload which is executing
> do_init_module() and is now at
> 
> fail_free_freeinit:
>         kfree(freeinit);
> fail:
>         /* Try to protect us from buggy refcounters. */
>         mod->state = MODULE_STATE_GOING;
>         synchronize_rcu();
>         module_put(mod);
>         blocking_notifier_call_chain(&module_notify_list,
>                                      MODULE_STATE_GOING, mod);
>         klp_module_going(mod);
>         ftrace_release_mod(mod);
>         free_module(mod);
>         wake_up_all(&module_wq);
>         return ret;
> }
> 
> The 191 threads cannot schedule and the system is effectively stuck.  It *does*
> eventually free itself but in some cases it takes minutes to do so.
> 
> A simple fix for this is to, as I've done above, to add a timeout so that
> the threads can be scheduled which allows other processes to run.  

After taking a much closer look the above patch appears to be correct.  I am not
seeing any boot failures associated with it anywhere.  I would like to hear from
Heiko as to whether or not this works for him though.

P.