Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753935Ab3F1FyO (ORCPT ); Fri, 28 Jun 2013 01:54:14 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:37540 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752862Ab3F1FyK (ORCPT ); Fri, 28 Jun 2013 01:54:10 -0400 Message-ID: <51CD24E1.2030608@canonical.com> Date: Fri, 28 Jun 2013 07:53:37 +0200 From: Maarten Lankhorst User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7 MIME-Version: 1.0 To: Davidlohr Bueso CC: Ingo Molnar , Rik van Riel , LKML , Peter Zijlstra , Thomas Gleixner Subject: Re: [PATCH] mutex: do not unnecessarily deal with waiters References: <1369353543.1770.0.camel@buesod1.americas.hpqcorp.net> <20130627090016.GA4398@gmail.com> <1372383138.2072.42.camel@buesod1.americas.hpqcorp.net> In-Reply-To: <1372383138.2072.42.camel@buesod1.americas.hpqcorp.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6425 Lines: 196 Op 28-06-13 03:32, Davidlohr Bueso schreef: > On Thu, 2013-06-27 at 11:00 +0200, Ingo Molnar wrote: > [...] >> So I tried this out yesterday, but it interacted with the Wait/Wound >> patches in tip:core/mutexes. >> >> Maarten Lankhorst pointed out that if this patch is applied on top of the >> WW patches as-is, then we get this semantic merge conflict: >> >>>> @@ -340,6 +339,14 @@ slowpath: >>>> #endif >>>> spin_lock_mutex(&lock->wait_lock, flags); >>>> >>>> + /* once more, can we acquire the lock? */ >>>> + if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) { >>>> + lock_acquired(&lock->dep_map, ip); >>>> + mutex_set_owner(lock); >>>> + spin_unlock_mutex(&lock->wait_lock, flags); >>>> + goto done; >>>> + } >>>> >>> ^^^^^^^^^^^^^^ >>> >>> This part skips the whole if (!__builtin_constant_p(ww_ctx == NULL)) { >>> section with the wait_lock held. > I see what you mean, I hadn't really looked at the W/W patches. BTW > those __builtin_constant_p() calls are pretty ugly/annoying to read, > plus why the negation of the NULL check? Couldn't we just do something > like: It's to kill overhead.. ww_ctx == NULL is a constant only when the function is called with null as explicit parameter. So !__builtin_constant_p(ww_ctx == NULL) means that the function was called with a variable ww_ctx. > #define is_ww_ctx(x) (__builtin_constant_p(x)) > ... > if (is_ww_ctxt(ww_ctx)) { ... } > > > Anyway, so going back to the actual patch, we need a few cleanups in > __mutex_lock_common() before we can rebase this patch - otherwise we're > going to end up duplicating a lot of code (and the function is already > big enough): > > How about a new ww_mutex_set_context_slowpath() function that does the > w/w lock acquiring and wakes up any sleeping processes. We'd use this > function whenever we acquire the lock in the slowpath (with the > ->wait_lock taken): > > static __always_inline void > ww_mutex_set_context_slowpath(struct mutex *lock, > struct ww_acquire_ctx *ww_ctx, bool debug) > { > if (!__builtin_constant_p(ww_ctx == NULL)) { > struct mutex_waiter *cur; > struct ww_mutex *ww = container_of(lock, struct ww_mutex, base); > > /* > * This branch gets optimized out for the common case, > * and is only important for ww_mutex_lock. > */ > ww_mutex_lock_acquired(ww, ww_ctx); > ww->ctx = ww_ctx; > > /* > * Give any possible sleeping processes the chance to wake up, > * so they can recheck if they have to back off. > */ > list_for_each_entry(cur, &lock->wait_list, list) { > if (debug) > debug_mutex_wake_waiter(lock, cur); > wake_up_process(cur->task); > } > } > } > > In ww_mutex_set_context_fastpath() I'm a little confused with the > debug_mutex_wake_waiter() calls since we don't deal with debug in the > fast path (->wait_lock isn't held). So are these calls > correct/necessary? Well spotted, but in that case the !debug case mutex_wake_waiter gets optimized out anyway, so please don't add a conditional like that. > For ww_mutex_set_context_slowpath(), the 'debug' parameter would be > necessary since with this patch we avoid doing the debug_mutex on a > quick attempt to grab the lock, otherwise we do the slowpath debug, > waiters, etc. For instance: > > ... > slowpath: > #endif > spin_lock_mutex(&lock->wait_lock, flags); > /* once more, can we acquire the lock? */ > if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) { > lock_acquired(&lock->dep_map, ip); > mutex_set_owner(lock); > ww_mutex_set_context_fastpath(lock, ww_ctx, false); > spin_unlock_mutex(&lock->wait_lock, flags); > goto done; > } > ... > > lock_acquired(&lock->dep_map, ip); > /* got the lock - rejoice! */ > mutex_remove_waiter(lock, &waiter, current_thread_info()); > mutex_set_owner(lock); > ww_mutex_set_context_slowpath(lock, ww_ctx, true); > ... I used the power of goto's in my own fixed up version below, and reshuffled some calls a bit. Maybe you could verify if it's correct, and if it is use it as base? 8<--------- diff --git a/kernel/mutex.c b/kernel/mutex.c index e581ada..f93be1d 100644 --- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -486,8 +486,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, mutex_set_owner(lock); mspin_unlock(MLOCK(lock), &node); - preempt_enable(); - return 0; + goto done; } mspin_unlock(MLOCK(lock), &node); @@ -512,6 +511,10 @@ slowpath: #endif spin_lock_mutex(&lock->wait_lock, flags); + /* once more, can we acquire the lock? */ + if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) + goto skip_wait; + debug_mutex_lock_common(lock, &waiter); debug_mutex_add_waiter(lock, &waiter, task_thread_info(task)); @@ -519,9 +522,6 @@ slowpath: list_add_tail(&waiter.list, &lock->wait_list); waiter.task = task; - if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1)) - goto done; - lock_contended(&lock->dep_map, ip); for (;;) { @@ -535,7 +535,7 @@ slowpath: * other waiters: */ if (MUTEX_SHOW_NO_WAITER(lock) && - (atomic_xchg(&lock->count, -1) == 1)) + (atomic_xchg(&lock->count, -1) == 1)) break; /* @@ -560,11 +560,15 @@ slowpath: schedule_preempt_disabled(); spin_lock_mutex(&lock->wait_lock, flags); } + mutex_remove_waiter(lock, &waiter, current_thread_info()); + /* set it to 0 if there are no waiters left: */ + if (likely(list_empty(&lock->wait_list))) + atomic_set(&lock->count, 0); + debug_mutex_free_waiter(&waiter); -done: +skip_wait: + /* got the lock - cleanup and rejoice! */ lock_acquired(&lock->dep_map, ip); - /* got the lock - rejoice! */ - mutex_remove_waiter(lock, &waiter, current_thread_info()); mutex_set_owner(lock); if (!__builtin_constant_p(ww_ctx == NULL)) { @@ -591,15 +595,9 @@ done: } } - /* set it to 0 if there are no waiters left: */ - if (likely(list_empty(&lock->wait_list))) - atomic_set(&lock->count, 0); - spin_unlock_mutex(&lock->wait_lock, flags); - - debug_mutex_free_waiter(&waiter); +done: preempt_enable(); - return 0; err: -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/