From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756228AbbDIU6W (ORCPT ); Thu, 9 Apr 2015 16:58:22 -0400 Received: from mail-ig0-f180.google.com ([209.85.213.180]:35842 "EHLO mail-ig0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756121AbbDIU6R (ORCPT ); Thu, 9 Apr 2015 16:58:17 -0400 MIME-Version: 1.0 In-Reply-To: <20150409195849.GN6464@linux.vnet.ibm.com> References: <1428521960-5268-1-git-send-email-jason.low2@hp.com> <1428521960-5268-3-git-send-email-jason.low2@hp.com> <20150409053725.GB13871@gmail.com> <1428561611.3506.78.camel@j-VirtualBox> <20150409075311.GA4645@gmail.com> <20150409175652.GI6464@linux.vnet.ibm.com> <1428608618.12911.9.camel@j-VirtualBox> <20150409195849.GN6464@linux.vnet.ibm.com> Date: Thu, 9 Apr 2015 13:58:16 -0700 X-Google-Sender-Auth: _AzwJSUCFhUjvxai58tUP9mBLm0 Message-ID: Subject: Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() From: Jason Low To: Paul McKenney Cc: Jason Low , Linus Torvalds , Ingo Molnar , Peter Zijlstra , Davidlohr Bueso , Tim Chen , Aswin Chandramouleeswaran , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 9, 2015 at 12:58 PM, Paul E. McKenney wrote: > On Thu, Apr 09, 2015 at 12:43:38PM -0700, Jason Low wrote: >> On Thu, 2015-04-09 at 11:16 -0700, Linus Torvalds wrote: >> > On Thu, Apr 9, 2015 at 11:08 AM, Linus Torvalds >> > wrote: >> > > >> > > The pointer is a known-safe kernel pointer - it's just that it was >> > > "known safe" a few instructions ago, and might be rcu-free'd at any >> > > time. >> > >> > Actually, we could even do something like this: >> > >> > static inline int sem_owner_on_cpu(struct semaphore *sem, struct >> > task_struct *owner) >> > { >> > int on_cpu; >> > >> > #ifdef CONFIG_DEBUG_PAGEALLOC >> > rcu_read_lock(); >> > #endif >> > on_cpu = sem->owner == owner && owner->on_cpu; >> > #ifdef CONFIG_DEBUG_PAGEALLOC >> > rcu_read_unlock(); >> > #endif >> > return on_cpu; >> > } >> > >> > because we really don't need to hold the RCU lock over the whole loop, >> > we just need to validate that the semaphore owner still matches, and >> > if so, check that it's on_cpu. >> > >> > And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about performance >> > *at*all*. We will have worse performance problems than doing some RCU >> > read-locking inside the loop. >> > >> > And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care about >> > locking, since at worst we just access stale memory for one iteration. >> > >> > Hmm. It's not pretty, but neither is the current "let's just take a >> > rcu lock that we don't really need over a loop that doesn't have very >> > strict bounding". >> > >> > Comments? >> >> So that looks more similar to how the original code was where the >> rcu_read_lock() and rcu_read_unlock() was done inside the owner_running >> helper function (though without the CONFIG_DEBUG_PAGEALLOC), before >> commit 307bf9803f25 ("sched: Simplify mutex_spin_on_owner()") modified >> it to be done outside the loop. > > Another approach would be to post a timer before entering the spinloop, > and have the timer handler set the resched bit. Then the loop would > be bounded, safe, and would run at full speed. Though posting a timer, ect... would also add more overhead right?