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[23.128.96.18]) by mx.google.com with ESMTP id h8si5632092edk.97.2020.10.17.20.27.31; Sat, 17 Oct 2020 20:28:20 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2440056AbgJRAzO (ORCPT + 99 others); Sat, 17 Oct 2020 20:55:14 -0400 Received: from kvm5.telegraphics.com.au ([98.124.60.144]:39744 "EHLO kvm5.telegraphics.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2440050AbgJRAzN (ORCPT ); Sat, 17 Oct 2020 20:55:13 -0400 Received: from localhost (localhost.localdomain [127.0.0.1]) by kvm5.telegraphics.com.au (Postfix) with ESMTP id C51E82A7EE; Sat, 17 Oct 2020 20:55:06 -0400 (EDT) Date: Sun, 18 Oct 2020 11:54:54 +1100 (AEDT) From: Finn Thain To: Arnd Bergmann cc: "linux-kernel@vger.kernel.org" , Russell King , Tony Luck , Fenghua Yu , Greg Ungerer , Geert Uytterhoeven , Philip Blundell , Joshua Thompson , Sam Creasey , "James E.J. Bottomley" , Helge Deller , Thomas Gleixner , Daniel Lezcano , John Stultz , Stephen Boyd , Linus Walleij , linux-ia64@vger.kernel.org, Parisc List , linux-m68k , Linux ARM Subject: Re: [RFC 13/13] m68k: mac: convert to generic clockevent In-Reply-To: Message-ID: References: <20201008154651.1901126-1-arnd@arndb.de> <20201008154651.1901126-14-arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Oct 2020, Arnd Bergmann wrote: > On Thu, Oct 15, 2020 at 3:19 AM Finn Thain wrote: > > > > On Sat, 10 Oct 2020, Arnd Bergmann wrote: > > > > > > Perhaps patch 13 does not belong in this series (?). > > > > > > > > All m68k platforms will need conversion before the TODO can be removed > > > > from Documentation/features/time/clockevents/arch-support.txt. > > > > > > Yes, correct. I marked this patch as RFC instead of PATCH, as I'm just > > > trying to find out where it should be headed. I would hope the other > > > patches can just get merged. > > > > > > > I wonder whether we can improve support for your proposed configuration > > i.e. a system with no oneshot clockevent device. > > > > The 16 platforms you identified are not all in that category but I suspect > > that there are others which are (though they don't appear in this series > > because they already use GENERIC_CLOCKEVENTS). > > > > One useful optimization would be some way to elide oneshot clockevent > > support (perhaps with the help of Link Time Optimization). > > I think this already happens if one picks CONFIG_HZ_PERIODIC while > disabling HIGH_RES_TIMERS. In that case, CONFIG_TICK_ONESHOT > remains disabled. > That configuration still produces the same 5 KiB of bloat. I see that kernel/time/Kconfig has this -- # Core internal switch. Selected by NO_HZ_COMMON / HIGH_RES_TIMERS. This is # only related to the tick functionality. Oneshot clockevent devices # are supported independent of this. config TICK_ONESHOT bool But my question was really about both kinds of dead code (oneshot device support and oneshot tick support). Anyway, after playing with the code for a bit I don't see any easy way to reduce the growth in text size. > ... > > After looking at the chip documentation I don't think it's viable to > > use the hardware timers in the way I proposed. A VIA register access > > requires at least one full VIA clock cycle (about 1.3 us) which means > > register accesses themselves cause timing delays. They also make > > clocksource reads expensive. > > > > I think this rules out oneshot clockevent devices because if the > > system offered such a device it would preferentially get used as a > > tick device. > > > > So I think your approach (periodic clockevent device driven by the > > existing periodic tick interrupt) is best for this platform due to > > simplicity (not much code) and performance (good accuracy, no > > additional overhead). > > Yes, makes sense. I think the one remaining problem with the periodic > mode in this driver is that it can drop timer ticks when interrupts are > disabled for too long, while in oneshot mode there may be a way to know > how much time has passed since the last tick as long as the counter does > not overflow. Is there any benefit from adopting a oneshot tick (rather than periodic) if no clocksource is consulted when calculating the next interval? (I'm assuming NO_HZ is not in use, for reasons discussed below.) > I would agree that despite this oneshot mode is probably worse overall > for timekeeping if the register accesses introduce systematic errors. > It probably has to be tried. But consulting a VIA clocksource on every tick would be expensive on this platform, so if that was the only way to avoid cumulative errors, I'd probably just stick with the periodic tick. > ... > The arm/rpc timer seems to be roughly in the same category as most of > the m68k ones or the i8253 counter on a PC. It's possible that some of > them could use the same logic as drivers/clocksource/i8253.o as long as > there is any hardware oneshot mode. > There appear to be 15 platforms in that category. 4 have no clocksource besides the jiffies clocksource, meaning there's no practical alternative to using a periodic tick, like you did in your RFC patch: arch/m68k/apollo/config.c arch/m68k/q40/q40ints.c arch/m68k/sun3/sun3ints.c arch/m68k/sun3x/time.c The other 11 platforms in that category also have 'synthetic' clocksources derived from a timer reload interrupt. In 3 cases, the clocksource read method does not (or can not) check for a pending counter reload interrupt. For these also, I see no practical alternative to the approach you've taken in your RFC patch: arch/m68k/68000/timers.c arch/m68k/atari/time.c arch/m68k/coldfire/timers.c That leaves 8 platforms that have reliable clocksource devices which should be able to provide an accurate reading even in the presence of a dropped tick (due to drivers disabling interrupts for too long): arch/arm/mach-rpc/time.c arch/m68k/amiga/config.c arch/m68k/bvme6000/config.c arch/m68k/coldfire/sltimers.c arch/m68k/hp300/time.c arch/m68k/mac/via.c arch/m68k/mvme147/config.c arch/m68k/mvme16x/config.c But is there any reason to adopt a oneshot tick on any of these platforms, if NO_HZ won't eliminate the timer interrupt that's needed to run a 'synthetic' clocksource?