From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933405AbbBCKxZ (ORCPT ); Tue, 3 Feb 2015 05:53:25 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:42706 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753451AbbBCKxM (ORCPT ); Tue, 3 Feb 2015 05:53:12 -0500 Date: Tue, 3 Feb 2015 11:53:03 +0100 From: Peter Zijlstra To: Frederic Weisbecker Cc: Ingo Molnar , LKML , Steven Rostedt , Linus Torvalds Subject: Re: [PATCH 3/4] sched: Pull preemption disablement to __schedule() caller Message-ID: <20150203105303.GI26304@twins.programming.kicks-ass.net> References: <1422404652-29067-1-git-send-email-fweisbec@gmail.com> <1422404652-29067-4-git-send-email-fweisbec@gmail.com> <20150128155044.GJ23038@twins.programming.kicks-ass.net> <20150202175343.GD11054@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150202175343.GD11054@lerouge> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 02, 2015 at 06:53:45PM +0100, Frederic Weisbecker wrote: > It looks like preempt_count_add/inc() mostly imply entering a context that we want > to be seen right away (thus want barrier() after) and preempt_count_sub/dec() mostly > want previous work to be visible before re-enabling interrupt, preemption, etc... > (thus want barrier() before). > > So maybe these functions (the non-underscored ones) should imply a barrier() rather > than their callers (preempt_disable() and others). Inline functions instead of macros > would do the trick (if the headers hell let us do that). > > Note the underscored implementations are all inline currently so this happens to > work by chance for direct calls to preempt_count_add/sub() outside preempt_disable(). > If the non-underscored caller is turned into inline too I don't expect performance issues. > > What do you think, does it make sense? AFAIK inline does _not_ guarantee a compiler barrier, only an actual function call does. When inlining the compiler creates visibility into the 'call' and can avoid the constraint -- teh interweb seems to agree and also pointed out that 'pure' function calls, even when actual function calls, can avoid being a compiler barrier. The below blog seems to do a fair job of explaining things; in particular the 'implied compiler barriers' section is relevant here: http://preshing.com/20120625/memory-ordering-at-compile-time/ As it stands the difference between the non underscore and the underscore version is debug/tracing muck. The underscore ops are the raw operations without fancy bits on. I think I would prefer keeping it that way; this means that preempt_count_$op() is a pure op and when we want to build stuff with it like preempt_{en,dis}able() they add the extra semantics on top. In any case; if we make __schedule() noinline (I think that might make sense) that function call would itself imply the compiler barrier and something like: __preempt_count_add(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET); __schedule(); __preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET); Would actually be safe/correct. As it stands I think __schedule() would fail the GCC inline static criteria for being too large, but you never know, noinline guarantees it will not.