Messages in this thread |  | | Date | Tue, 21 Oct 2014 19:34:13 +0900 | From | Masami Hiramatsu <> | Subject | Re: Re: [RFC PATCH 5/5] module: Remove stop_machine from module unloading |
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(2014/10/13 13:40), Rusty Russell wrote: > 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.
OK, I might be thinking too much about performance. I'll drop it :)
> 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.
OK, I'll do that.
> >> +/* >> + * 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?
Would you mean just replacing atomic_inc_not_zero() with atomic_add_unless()?
> >> + >> /* 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!
-- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@hitachi.com
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