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

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Date:	Wed, 2 Oct 2013 13:27:34 -0400
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Stephen Boyd <sboyd@...eaurora.org>
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 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.

Regards,
Santosh

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