From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1033914AbdAEQJE (ORCPT ); Thu, 5 Jan 2017 11:09:04 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:56370 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934348AbdAEQIf (ORCPT ); Thu, 5 Jan 2017 11:08:35 -0500 Date: Thu, 5 Jan 2017 17:08:34 +0100 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , linux-kernel@vger.kernel.org, Steven Rostedt , Daniel Bristot de Oliveira Subject: Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks Message-ID: <20170105160834.GB3107@twins.programming.kicks-ass.net> References: <1483466430-8028-1-git-send-email-longman@redhat.com> <20170104124929.GI25813@worktop.programming.kicks-ass.net> <57c0a6e1-1fab-b8d4-2a68-02cc1ebf7ce6@redhat.com> <20170105094439.GD3093@worktop> <342e0af6-43b1-76ea-0f0b-55087dfec96c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <342e0af6-43b1-76ea-0f0b-55087dfec96c@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 05, 2017 at 10:55:55AM -0500, Waiman Long wrote: > What I am saying that we don't need to change spinlock to rtmutex in a > -RT kernel. Instead, we can use rtqspinlock for this purpose. All the > sleeping locks will still be converted to rtmutex. No-no-no.. > Conversion of rtmutex does allow forced CPU preemption when there is a > need for that. What rtqspinlock can provide is voluntary preemption > where the lock waiters explicitly yield the CPU while waiting for the > lock. I use the need_resched() to detect if CPU yielding is necessary. > However, if the CPU was in a preempt disabled region before the > spin_lock() call, we can't yield the CPU. The only way is to raise its > priority and try to get the lock ASAP. And here you've lost your finger because the saw-blade didn't stop in time. RT very fundamentally relies on the spinlock->rtmutex conversion to allow preempting things when a higher priority task comes along. A spinlock, of any kind, requires having preemption disabled while holding the lock. If the critical section is of unbounded latency, you have unbounded preemption latency and RT is no more. Its not about PI on contention, although that helps inversion scenarios. Its about allowing preemption, which fundamentally requires a sleeping lock to be used. Many of the spinlock sections of mainline are not well behaved in an RT sense and therefore must not disable preemption. Similar for the IRQ disable regions and hence we have the whole threaded interrupt stuff. Please stop writing code and read up on things..