Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754574AbbBZVeC (ORCPT ); Thu, 26 Feb 2015 16:34:02 -0500 Received: from mail-qg0-f54.google.com ([209.85.192.54]:54092 "EHLO mail-qg0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753685AbbBZVd7 (ORCPT ); Thu, 26 Feb 2015 16:33:59 -0500 Date: Thu, 26 Feb 2015 16:33:55 -0500 (EST) From: Nicolas Pitre To: Daniel Thompson cc: Thomas Gleixner , Jason Cooper , Russell King , Will Deacon , Catalin Marinas , Marc Zyngier , Stephen Boyd , John Stultz , Steven Rostedt , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, patches@linaro.org, linaro-kernel@lists.linaro.org, Sumit Semwal , Dirk Behme , Daniel Drake , Dmitry Pervushin , Tim Sander Subject: Re: [PATCH 3.19-rc6 v16 1/6] irqchip: gic: Optimize locking in gic_raise_softirq In-Reply-To: <1424984740.21020.11.camel@linaro.org> Message-ID: References: <1422022952-31552-1-git-send-email-daniel.thompson@linaro.org> <1422990417-1783-1-git-send-email-daniel.thompson@linaro.org> <1422990417-1783-2-git-send-email-daniel.thompson@linaro.org> <1424984740.21020.11.camel@linaro.org> User-Agent: Alpine 2.11 (LFD 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2279 Lines: 70 On Thu, 26 Feb 2015, Daniel Thompson wrote: > On Thu, 2015-02-26 at 15:31 -0500, Nicolas Pitre wrote: > > On Tue, 3 Feb 2015, Daniel Thompson wrote: > > > > > Currently gic_raise_softirq() is locked using upon irq_controller_lock. > > > This lock is primarily used to make register read-modify-write sequences > > > atomic but gic_raise_softirq() uses it instead to ensure that the > > > big.LITTLE migration logic can figure out when it is safe to migrate > > > interrupts between physical cores. > > > > > > This is sub-optimal in closely related ways: > > > > > > 1. No locking at all is required on systems where the b.L switcher is > > > not configured. > > > > ACK > > > > > 2. Finer grain locking can be used on systems where the b.L switcher is > > > present. > > > > NAK > > > > Consider this sequence: > > > > CPU 1 CPU 2 > > ----- ----- > > gic_raise_softirq() gic_migrate_target() > > bl_migration_lock() [OK] > > [...] [...] > > map |= gic_cpu_map[cpu]; bl_migration_lock() [contended] > > bl_migration_unlock(flags); bl_migration_lock() [OK] > > gic_cpu_map[cpu] = 1 << new_cpu_id; > > bl_migration_unlock(flags); > > [...] > > (migrate pending IPI from old CPU) > > writel_relaxed(map to GIC_DIST_SOFTINT); > > Isn't this solved inside gic_raise_softirq? How can the writel_relaxed() > escape from the critical section and happen at the end of the sequence? Hmmm... blah. OK I obviously can't read today. The patch is fine of course. > > Oh, and a minor nit: > > > > > + * This lock is used by the big.LITTLE migration code to ensure no IPIs > > > + * can be pended on the old core after the map has been updated. > > > + */ > > > +#ifdef CONFIG_BL_SWITCHER > > > +static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock); > > > + > > > +static inline void bl_migration_lock(unsigned long *flags) > > > > Please name it gic_migration_lock. "bl_migration_lock" is a bit too > > generic in this context. > > I'll change this. Good. You may add my ACK. Nicolas -- 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/