linux-kernel - Re: [RESEND PATCH 3/3] x86/vmware: Add paravirt sched clock

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Message-ID: <alpine.DEB.2.20.1610272345510.4913@nanos>
Date:   Fri, 28 Oct 2016 00:10:12 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexey Makhalov <amakhalov@...are.com>
cc:     corbet@....net, akataria@...are.com, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, pv-drivers@...are.com
Subject: Re: [RESEND PATCH 3/3] x86/vmware: Add paravirt sched clock

On Thu, 27 Oct 2016, Alexey Makhalov wrote:

> Set pv_time_ops.sched_clock to vmware_sched_clock(). 

Please do not describe WHAT the patch does, describe why. Describe the
problem you are solving. I can see from the patch

> +		pv_time_ops.sched_clock = vmware_sched_clock;

that you set pv_time_ops.sched_clock to vmware_sched_clock().

> It is simplified
> version of native_sched_clock() without ring buffer of mult/shift/offset
> triplets and preempt toggling.

-ENOPARSE

> Since VMware hypervisor provides constant tsc we can use constant
> mult/shift/offset triplet calculated at boot time.

So now you start to explain something which is understandable

> no-vmw-sched-clock kernel parameter is added to disable the paravirt
> sched clock.

I give you another example:

  The default sched_clock() implementation is native_sched_clock(). It
  contains code to handle non constant frequency TSCs, which creates
  overhead for systems with constant frequency TSCs.

  The vmware hypervisor guarantees a constant frequency TSC, so
  native_sched_clock() is not required and slower than a dedicated function
  which operates with one time calculated conversion factors.

  Calculate the conversion factors at boot time from the tsc frequency and
  install an optimized sched_clock() function via paravirt ops.

  The paravirtualized clock can be disabled on the kernel command line with
  the new 'no-vmw-sched-clock' option.

Can you see the difference and can you spot the structure similar to the
example I gave you before?

> +static unsigned long long vmware_sched_clock(void)
> +{
> +	unsigned long long ns;
> +
> +	ns = mul_u64_u32_shr(rdtsc(), vmware_cyc2ns.cyc2ns_mul,
> +			     vmware_cyc2ns.cyc2ns_shift);
> +	ns -= vmware_cyc2ns.cyc2ns_offset;
> +	return ns;
> +}
> +
>  static void __init vmware_paravirt_ops_setup(void)
>  {
>  	pv_info.name = "VMware hypervisor";
>  	pv_cpu_ops.io_delay = paravirt_nop;
> +
> +	if (vmware_tsc_khz && vmw_sched_clock) {
> +		unsigned long long tsc_now = rdtsc();
> +
> +		clocks_calc_mult_shift(&vmware_cyc2ns.cyc2ns_mul,
> +				       &vmware_cyc2ns.cyc2ns_shift,
> +				       vmware_tsc_khz,
> +				       NSEC_PER_MSEC, 0);
> +		vmware_cyc2ns.cyc2ns_offset =
> +			mul_u64_u32_shr(tsc_now, vmware_cyc2ns.cyc2ns_mul,
> +					vmware_cyc2ns.cyc2ns_shift);
> +
> +		pv_time_ops.sched_clock = vmware_sched_clock;
> +		pr_info("using sched offset of %llu ns\n",
> +			vmware_cyc2ns.cyc2ns_offset);

If you either do:

	if (!vmware_tsc_khz || !vmw_sched_clock)
		return;

or

	if (vmware_tsc_khz && vmw_sched_clock)
		setup_sched_clock();

and split out the code into a seperate function then you spare one
indentation level and some of these hard to read line breaks.

Hint:

static void setup_sched_clock(void)
{
	struct cyc2ns_data *d = &vmware_cyc2ns;

	clocks_calc_mult_shift(&d->cyc2ns_mul, &d->cyc32ns_shift,
			       vmware_tsc_khz, NSEC_PER_MSEC, 0);

reduces the lenght of the arguments significantly and makes this stuff sane
to read.

Thanks,

	tglx