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[23.128.96.18]) by mx.google.com with ESMTP id d23si4265354ejd.151.2020.10.30.06.13.59; Fri, 30 Oct 2020 06:14:23 -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 S1726078AbgJ3NMf (ORCPT + 99 others); Fri, 30 Oct 2020 09:12:35 -0400 Received: from mail.kernel.org ([198.145.29.99]:37264 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725999AbgJ3NMf (ORCPT ); Fri, 30 Oct 2020 09:12:35 -0400 Received: from [192.168.0.244] (118-211-2-196.tpgi.com.au [118.211.2.196]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1167B206FA; Fri, 30 Oct 2020 13:12:27 +0000 (UTC) Subject: Re: [RFC 13/13] m68k: mac: convert to generic clockevent To: Finn Thain , Arnd Bergmann Cc: "linux-kernel@vger.kernel.org" , Russell King , Tony Luck , Fenghua Yu , 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 References: <20201008154651.1901126-1-arnd@arndb.de> <20201008154651.1901126-14-arnd@arndb.de> From: Greg Ungerer Message-ID: <580c3542-92cc-7e33-a43d-bf6a68134a46@linux-m68k.org> Date: Fri, 30 Oct 2020 23:12:29 +1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/10/20 10:41 am, Finn Thain wrote: > On Fri, 23 Oct 2020, Arnd Bergmann wrote: >> On Sun, Oct 18, 2020 at 2:55 AM Finn Thain wrote: >>> 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: >>> >>> 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. >> >> Did you look more deeply into where those 5KB are? Is this just the code >> in kernel/time/{clockevents,tick-common}.c and the added platform >> specific bits, or is there something more? > > What I did was to list the relevant functions using scripts/bloat-o-meter > and tried stubbing out some code related to oneshot clockevent devices. I > didn't find any low fruit and don't plan to pursue that without the help > of LTO. > >> I suppose the sysfs interface and the clockevents_update_freq() logic >> are not really needed on m68k, but it wouldn't make much sense to split >> those out either. >> >> How does the 5KB bloat compare to the average bloat we get from one >> release to the next? Geert has been collecting statistics for this. >> > > Perhaps that 5 KB is justified by gaining the hrtimers feature... hard to > say; it's never been available on these platforms. I can see the value in > it though. > >>>> 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.) >> >> If the clocksource does not set CLOCK_SOURCE_IS_CONTINOUS, the kernel >> will keep using periodic timers and not allow hrtimers. >> > > IIUC, when HIGH_RES_TIMERS=y, the kernel will enable hrtimers only if the > platform provides both a continuous clocksource device and a oneshot > clockevent device. However, the "jiffies" clocksource does not set > CLOCK_SOURCE_IS_CONTINOUS and neither does the one in > arch/arm/mach-rpc/time.c. > > When HIGH_RES_TIMERS=n and NO_HZ_COMMON=n (which is presently the case for > all platforms with GENERIC_CLOCKEVENTS=n) there's no use for a oneshot > clockevent device, right? > > It seems likely that NO_HZ_COMMON=n because the clocksources on these > platforms produce a periodic interrupt regardless (e.g. > clocksource/i8253.c, arm/rpc, m68k platform timers etc.). > > Finally, HIGH_RES_TIMERS=n seems likely if the only clocksource is > unreliable (e.g. because it loses time due to interrupt priorities). There > may be a few of platforms in this category (Mac, Atari?). > >>>> 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. >> >> I'm sure there is a tradeoff somewhere. Without hrtimers, some events >> will take longer when they have to wait for the next tick, and using >> NO_HZ_FULL can help help make things faster on some workloads. >> > > Yes, such a tradeoff is discussed in drivers/iio/adc/ep93xx_adc.c. > > But OTOH, Documentation/timers/timers-howto.rst says, > > On slower systems, (embedded, OR perhaps a speed-stepped PC!) the > overhead of setting up the hrtimers for usleep *may* not be worth it > > I guess it has to be tried. > >> ... >>> 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 >> >> Agreed. It's possible there is a way, but I don't see one either. >> > > For arch/m68k/68000/timers.c, I suppose we may be able to check for the > TMR1 bit in the Interrupt Status Register at 0xFFFFF30C or the COMP bit in > the Timer Status Register at 0xFFFFF60A. But testing that patch could be > difficult. > > I expect that equivalent flags are available in Coldfire registers, making > it possible to improve upon mcftmr_read_clk() and m68328_read_clk() if > need be -- that is, if it turns out that the clocksource interrupt was > subject to higher priority IRQs that would slow down the clocksource or > defeat its monotonicity. > > The other difficulty is a lack of hardware timers. There's only one timer > on MC68EZ328. On Atari, for now only Timer C is available though Michael > has said that it would be possible to free up Timer D. Some Coldfire chips > have only 2 timers and the second timer seems to be allocated to > profiling. FWIW, I would have no problem with ditching the profiling clock, and using both on ColdFire platforms that have this. I doubt anyone has used that profiling setup in a _very_ long time. Regards Greg