linux-kernel - Re: [PATCH] sched_clock: fix postinit no sched

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Message-ID: <524C5B10.20006@codeaurora.org>
Date:	Wed, 02 Oct 2013 10:42:40 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
CC:	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"arm@...nel.org" <arm@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH] sched_clock: fix postinit no sched_clock function check

On 10/02/13 10:27, Santosh Shilimkar wrote:
> On Wednesday 02 October 2013 01:22 PM, Stephen Boyd wrote:
>> On 10/02/13 10:14, Santosh Shilimkar wrote:
>>> On Wednesday 02 October 2013 01:09 PM, Will Deacon wrote:
>>>> On Wed, Oct 02, 2013 at 05:55:28PM +0100, Santosh Shilimkar wrote:
>>>>> The sched_clock code uses 2 levels of function pointers, sched_clock_func()
>>>>> and read_sched_clock() but the no sched_clock check in postinit() just
>>>>> checks read_sched_clock().
>>>>>
>>>>> This leads to kernel falling back to jiffy based sched clock even in
>>>>> presence of sched_clock_func() which is not desirable.
>>>>>
>>>>> Fix the postinit() check to avoid the issue. Probably the issue is hidden
>>>>> so far on most of the arm SOCs because of already existing sched_clock
>>>>> registrations apart from arch_timer sched_clock. One can reproduce the
>>>>> issue by just have arch_timer as sched_clock
>>>> Isn't this just an issue with the arch timer driver not calling
>>>> setup_sched_clock? Instead, we munge around with sched_clock_func directly,
>>>> which doesn't appear to be the way anybody else deals with this.
>>>>
>>> I thought about that option as well but was not sure since even in that case
>>> the check is not complete. We just ensure that function is popullated.
>> Yes, nothing is actually broken because sched_clock_func() won't try to
>> use the jiffy based read_sched_clock() function. I'm not sure we
>> actually need this patch besides to remove a useless timer that updates
>> the jiffy epoch. Can we wait until my 64-bit sched_clock patch series
>> lands in 3.13? It looks like I still need an ack from Will or Catalin on
>> the architected timer patch before the clocksource folks pick it up.
>>
> Really... I have not created patch out of fun.
> Its broken on my keystone machine at least where the sched_clock is
> falling back on jiffy based sched_clock even in presence of arch_timer
> sched_clock.

How is that possible? sched_clock_func is only assigned by
arch/arm/kernel/arch_timer.c when the architected timer is detected and
sched_clock() in kernel/time/sched_clock.c calls that function pointer
unconditionally. The only way I see this happening is if the architected
timer rate is zero. I agree we will get two lines in the dmesg about
sched_clock and its not very clear which one is being used.

-- 
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