linux-kernel - Re: [PATCH v2 04/13] arm64/arch_timer: Provide noinstr sched_clock

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Message-ID: <xhsmhsfb3odht.mognet@vschneid.remote.csb>
Date:   Wed, 07 Jun 2023 09:58:22 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH v2 04/13] arm64/arch_timer: Provide noinstr
 sched_clock_read() functions

On 02/06/23 13:54, Peter Zijlstra wrote:
> On Wed, May 24, 2023 at 05:40:47PM +0100, Valentin Schneider wrote:
>>
>> So this bit sent me on a little spelunking session :-)
>>
>> From a control flow perspective the initialization isn't required, but then
>> I looked into the comment and found it comes from the
>> arch_timer_read_counter() definition... Which itself doesn't get used by
>> sched_clock() until the sched_clock_register() below!
>>
>> So AFAICT that comment was true as of
>>
>>   220069945b29 ("clocksource: arch_timer: Add support for memory mapped timers")
>>
>> but not after a commit that came 2 months later:
>>
>>   65cd4f6c99c1 ("arch_timer: Move to generic sched_clock framework")
>>
>> which IIUC made arm/arm64 follow the default approach of using the
>> jiffy-based sched_clock() before probing DT/ACPI and registering a "proper"
>> sched_clock.
>>
>> All of that to say: the comment about arch_timer_read_counter() vs early
>> sched_clock() doesn't apply anymore, but I think we need to keep its
>> initalization around for stuff like get_cycles(). This initialization here
>> should be OK to put to the bin, though.
>
> Something like the below folded in then?
>

Much better, thank you!