From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752286AbdJSIWX (ORCPT ); Thu, 19 Oct 2017 04:22:23 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:60107 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751908AbdJSIWT (ORCPT ); Thu, 19 Oct 2017 04:22:19 -0400 Date: Thu, 19 Oct 2017 10:22:11 +0200 (CEST) From: Thomas Gleixner To: Matt Redfearn cc: James Hogan , 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: 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 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 19 Oct 2017, Matt Redfearn wrote: > On 18/10/17 21:34, Thomas Gleixner wrote: > > On Wed, 11 Oct 2017, Matt Redfearn wrote: > > > 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. > > This is an issue because in some cases (hrtimer_reprogram -> > clockevents_program_event -> clockevents_program_min_delta, when > CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST=n) there is no retry performed in the > case of -ETIME. There has been a patch pending for some time > https://patchwork.kernel.org/patch/8909491/ which ought to address this and > retry in the case of an event in the past on this call path. But in the > meantime this patch vastly improves the situation. I somehow missed that one. Care to repost so we get that solved at the place where it should be solved. Thanks, tglx