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

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Message-ID: <524C565C.8010709@codeaurora.org>
Date:	Wed, 02 Oct 2013 10:22:36 -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 10:14, 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.

-- 
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