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
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