From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751993AbdJRUfC (ORCPT ); Wed, 18 Oct 2017 16:35:02 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:58651 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751516AbdJRUe6 (ORCPT ); Wed, 18 Oct 2017 16:34:58 -0400 Date: Wed, 18 Oct 2017 22:34:50 +0200 (CEST) From: Thomas Gleixner To: Matt Redfearn cc: Daniel Lezcano , linux-mips@linux-mips.org, Matt Redfearn , "# v3 . 19 +" , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] clocksource/mips-gic-timer: Fix rcu_sched timeouts from multithreading In-Reply-To: <1507730474-8577-1-git-send-email-matt.redfearn@mips.com> Message-ID: References: <1507730474-8577-1-git-send-email-matt.redfearn@mips.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 11 Oct 2017, Matt Redfearn wrote: > When the MIPS GIC clockevent code was written, it appears to have > inherited the 0x300 cycle min delta from the MIPS CPU timer driver. This > is suboptimal for two reasons. > > Firstly, the CPU timer counts once every other cycle (i.e. half the > clock rate). The GIC counts once per clock. Assuming that the GIC and > CPU share the same clock this means the GIC is counting twice as fast, > and so the min delta should be (at least) doubled. Fix this by doubling > the min delta to 0x600. > > Secondly, the fixed min delta ignores the fact that with MIPS > multithreading active, execution resource within a core is shared > between the hardware threads within that core. An inconvenienly timed > switch of executing thread within gic_next_event, between the read and > write of updated count, can result in the CPU writing an event in the > past, and subsequently not receiving a tick interrupt until the counter > wraps. This stalls the CPU from the RCU scheduler. Other CPUs detect > this and print rcu_sched timeout messages in the kernel log. It can > lead to other issues as well if the CPU is holding locks or other > resources at the point at which it stalls. Fix this by scaling the min > delta for the timer based on the number of threads in the core > (smp_num_siblings). This accounts for the greater average runtime of > CPUs within a multithreading core. I don't understand why this is not catched by the check at the end of the next_event() function: res = ((int)(gic_read_count() - cnt) >= 0) ? -ETIME : 0; Btw, the local_irq_save() in this function is pointless as this function is always called with interrupts disabled from the core code. Thanks, tglx