Messages in this thread | | | Subject | Re: [PATCH v3] kernel/module: Reschedule while waiting for modules to finish loading | From | Prarit Bhargava <> | Date | Thu, 2 May 2019 13:46:16 -0400 |
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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.
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