linux-kernel - Re: [PATCH v3 7/7] clocksource: mips-gic-timer: Set limitations on clocksource/sched-clocks usage

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Message-ID: <20200518144552.bjlddhuospgmer4j@mobilestation>
Date:   Mon, 18 May 2020 17:45:52 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, <linux-mips@...r.kernel.org>,
        <linux-rtc@...r.kernel.org>, <devicetree@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] clocksource: mips-gic-timer: Set limitations on
 clocksource/sched-clocks usage

On Mon, May 18, 2020 at 05:40:46PM +0300, Serge Semin wrote:
> Daniel,
> 
> On Mon, May 18, 2020 at 03:59:16PM +0200, Daniel Lezcano wrote:
> > On 16/05/2020 14:16, Serge Semin wrote:
> > > Hello Daniel,
> > > 
> > > Thanks for your comment. My response is below.
> > > 
> > > On Fri, May 15, 2020 at 07:10:04PM +0200, Daniel Lezcano wrote:
> > >> On Thu, May 07, 2020 at 12:41:07AM +0300, Serge Semin wrote:
> > >>> Currently neither clocksource nor scheduler clock kernel framework
> > >>> support the clocks with variable frequency. Needless to say how many
> > >>> problems may cause the sudden base clocks frequency change. In a
> > >>> simplest case the system time will either slow down or speed up.
> > >>> Since on CM2.5 and earlier MIPS GIC timer is synchronously clocked
> > >>> with CPU we must set some limitations on using it for these frameworks
> > >>> if CPU frequency may change. First of all it's not safe to have the
> > >>> MIPS GIC used for scheduler timings. So we shouldn't proceed with
> > >>> the clocks registration in the sched-subsystem. Secondly we must
> > >>> significantly decrease the MIPS GIC clocksource rating. This will let
> > >>> the system to use it only as a last resort.
> > >>>
> > >>> Note CM3.x-based systems may also experience the problems with MIPS GIC
> > >>> if the CPU-frequency change is activated for the whole CPU cluster
> > >>> instead of using the individual CPC core clocks divider.
> > >>
> > >> May be there is no alternative but the code looks a bit hacksih. Isn't possible
> > >> to do something with the sched_mark_unstable?
> > >>
> > >> Or just not use the timer at all ?
> > > 
> > > Not using the timer might be better, but not that good alternative either
> > > especially in our case due to very slow external timer. Me and Thomas
> > > Bogendoerfer discussed the similar commit I've provided to the csrc-r4k driver
> > > available on MIPS:
> > > https://lkml.org/lkml/2020/5/11/576
> > > 
> > > To cut it short, you are right. The solution with using clocksource_mark_unstable()
> > > is better alternative spied up in x86 tsc implementation. I'll use a similar
> > > approach here and submit the updated patch in v3.
> > > 
> > > Could you please proceed with the rest of the series review? I'd like to send
> > > the next version with as many comments taken into account as possible. The
> > > patchset has been submitted a while ago, but except Rob noone have had any
> > > comments.(
> > 
> > For me other patches are ok.
> > 
> > I can apply patches 1, 2, 4, 5, 6
> > 
> > Will remain patches 3 et 7
> 
> That's be great! Thanks. Is patch 3 supposed to be merged in by Rob or by you?
> I don't see one being in the Rob's repo. He might be waiting for you
> acknowledgment or something.

Ah, it's about the MIPS GIC IRQchip bindings conversion. No questions tabout
patch 3then. I'll ask Thomas about that patch status.

-Sergey 

> 
> I'll send the updated patch 3 shortly today.
> 
> -Sergey
> 
> > 
> > 
> > -- 
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