linux-kernel - Re: [PATCH v2 08/10] clocksource/drivers/hyper-v: Handle sched

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Date:   Tue, 2 Mar 2021 14:34:12 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Michael Kelley <mikelley@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 08/10] clocksource/drivers/hyper-v: Handle sched_clock
 differences inline

On 02/03/2021 02:38, Michael Kelley wrote:
> From: Daniel Lezcano <daniel.lezcano@...aro.org> Sent: Monday, March 1, 2021 6:25 AM
>>
>> On 01/03/2021 02:15, Michael Kelley wrote:
>>> While the Hyper-V Reference TSC code is architecture neutral, the
>>> pv_ops.time.sched_clock() function is implemented for x86/x64, but not
>>> for ARM64. Current code calls a utility function under arch/x86 (and
>>> coming, under arch/arm64) to handle the difference.
>>>
>>> Change this approach to handle the difference inline based on whether
>>> GENERIC_SCHED_CLOCK is present.  The new approach removes code under
>>> arch/* since the difference is tied more to the specifics of the Linux
>>> implementation than to the architecture.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
>>> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
>>> ---
>>
>> [ ... ]
>>
>>> +/*
>>> + * Reference to pv_ops must be inline so objtool
>>> + * detection of noinstr violations can work correctly.
>>> + */
>>> +static __always_inline void hv_setup_sched_clock(void *sched_clock)
>>> +{
>>> +#ifdef CONFIG_GENERIC_SCHED_CLOCK
>>> +	/*
>>> +	 * We're on an architecture with generic sched clock (not x86/x64).
>>> +	 * The Hyper-V sched clock read function returns nanoseconds, not
>>> +	 * the normal 100ns units of the Hyper-V synthetic clock.
>>> +	 */
>>> +	sched_clock_register(sched_clock, 64, NSEC_PER_SEC);
>>> +#else
>>> +#ifdef CONFIG_PARAVIRT
>>> +	/* We're on x86/x64 *and* using PV ops */
>>> +	pv_ops.time.sched_clock = sched_clock;
>>> +#endif
>>> +#endif
>>> +}
>> Please refer to:
>>
>> Documentation/process/coding-style.rst
>>
>> Section 21)
>>
>> [ ... ]
>>
>> Prefer to compile out entire functions, rather than portions of
>> functions or portions of expressions.
>>
>> [ ... ]
>>
> 
> OK.  I'll rework the #ifdef in v3 of the patch set.  Is the following
> the preferred approach?

Yes but with an indentation and comment to describe the section end.

eg.

#ifdef A
#else
# ifdef B
...
# else
# endif /* B */
#endif /* A */

> #ifdef CONFIG_GENERIC_SCHED_CLOCK
> static __always_inline void hv_setup_sched_clock(void *sched_clock)
> {
> 	sched_clock_register(sched_clock, 64 NSEC_PER_SEC);
> }
> #else
> #ifdef CONFIG_PARAVIRT
> static __always_inline void hv_setup_sched_clock(void *sched_clock)
> {
> 	pv_ops.time.sched_clock = sched_clock:
> }
> #else
> static __always_inline void hv_setup_sched_clock(void *sched_clock) {}
> #endif
> #endif

Or

#ifdef CONFIG_GENERIC_SCHED_CLOCK
...
#elif defined CONFIG_PARAVIRT
...
#else /* !CONFIG_GENERIC_SCHED_CLOCK && !CONFIG_PARAVIRT */
...
#endif /* CONFIG_GENERIC_SCHED_CLOCK */



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