Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [RFC PATCH 5/5] module: Remove stop_machine from module unloading | Date | Mon, 13 Oct 2014 15:10:27 +1030 |
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Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> writes: > Remove stop_machine from module unloading by replacing module_ref > with atomic_t. Note that this can cause a performance regression > on big-SMP machine by direct memory access. For those machines, > you can lockdwon all modules. Since the lockdown skips reference > counting, it'll be more scalable than per-cpu module_ref counters.
Sorry for the delay; I didn't get to this before I left, and then I was away for 3 weeks vacation.
First, I agree you should drop the MODULE_STATE_LOCKUP patch. While I can't audit every try_module_get() call, I did put an mdelay(100) in there and did a quick boot for any obvious slowdown.
Second, this patch should be split into two parts. The first would simply replace module_ref with atomic_t (a significant simplification), the second would get rid of stop machine.
> +/* > + * MODULE_REF_BASE must be 1, since we use atomic_inc_not_zero() for > + * recovering refcnt (see try_release_module_ref() ). > + */ > +#define MODULE_REF_BASE 1
True, but we could use atomic_add_unless() instead, and make this completely generic, right?
> + > /* Init the unload section of the module. */ > static int module_unload_init(struct module *mod) > { > - mod->refptr = alloc_percpu(struct module_ref); > - if (!mod->refptr) > - return -ENOMEM; > + /* > + * Initialize reference counter to MODULE_REF_BASE. > + * refcnt == 0 means module is going. > + */ > + atomic_set(&mod->refcnt, MODULE_REF_BASE); > > INIT_LIST_HEAD(&mod->source_list); > INIT_LIST_HEAD(&mod->target_list); > > /* Hold reference count during initialization. */ > - raw_cpu_write(mod->refptr->incs, 1); > + atomic_inc(&mod->refcnt); > > return 0; > } > @@ -721,8 +728,6 @@ static void module_unload_free(struct module *mod) > kfree(use); > } > mutex_unlock(&module_mutex); > - > - free_percpu(mod->refptr); > } > > #ifdef CONFIG_MODULE_FORCE_UNLOAD > @@ -740,60 +745,38 @@ static inline int try_force_unload(unsigned int flags) > } > #endif /* CONFIG_MODULE_FORCE_UNLOAD */ > > -struct stopref > +/* Try to release refcount of module, 0 means success. */ > +static int try_release_module_ref(struct module *mod) > { > - struct module *mod; > - int flags; > - int *forced; > -}; > + int ret; > > -/* Whole machine is stopped with interrupts off when this runs. */ > -static int __try_stop_module(void *_sref) > -{ > - struct stopref *sref = _sref; > + /* Try to decrement refcnt which we set at loading */ > + ret = atomic_sub_return(MODULE_REF_BASE, &mod->refcnt); > + if (ret) > + /* Someone can put this right now, recover with checking */ > + ret = atomic_inc_not_zero(&mod->refcnt); > + > + return ret; > +}
This is very clever! I really like it.
Thanks, Rusty.
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