On Tue, Feb 14, 2023 at 04:19:13PM +0000, Michael Kelley (LINUX) wrote:
> From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
> >
> > And have it preset.
> > This change allows to significantly reduce time to bring up guest SMP
> > configuration as well as make sure the guest won't get inaccurate
> > calibration results due to "noisy neighbour" situation.
> >
> > Below are the numbers for 16 VCPU guest before the patch (~1300 msec)
> >
> > [ 0.562938] x86: Booting SMP configuration:
> > ...
> > [ 1.859447] smp: Brought up 1 node, 16 CPUs
> >
> > and after the patch (~130 msec):
> >
> > [ 0.445079] x86: Booting SMP configuration:
> > ...
> > [ 0.575035] smp: Brought up 1 node, 16 CPUs
> >
> > This change is inspired by commit 0293615f3fb9 ("x86: KVM guest: use
> > paravirt function to calculate cpu khz").
>
> This patch has been nagging at me a bit, and I finally did some further
> checking. Looking at Linux guests on local Hyper-V and in Azure, I see
> a dmesg output line like this during boot:
>
> Calibrating delay loop (skipped), value calculated using timer frequency.. 5187.81 BogoMIPS (lpj=2593905)
>
> We're already skipping the delay loop calculation because lpj_fine
> is set in tsc_init(), using the results of get_loops_per_jiffy(). The
> latter does exactly the same calculation as hv_preset_lpj() in
> this patch.
>
> Is this patch arising from an environment where tsc_init() is
> skipped for some reason? Just trying to make sure we fully
> when this patch is applicable, and when not.
>
The problem here is a bit different: "lpj_fine" is considered only for
the boot CPU (from init/calibrate.c):
} else if ((!printed) && lpj_fine) {
lpj = lpj_fine;
pr_info("Calibrating delay loop (skipped), "
"value calculated using timer frequency.. ");
while all the secondary ones use the timer to calibrate.
With this change lpj_preset will be used for all cores (from
init/calbrate.c):
} else if (preset_lpj) {
lpj = preset_lpj;
if (!printed)
pr_info("Calibrating delay loop (skipped) "
"preset value.. ");
This lofic with lpj_fine comes from commit 3da757daf86e ("x86: use
cpu_khz for loops_per_jiffy calculation"), where the commit messages
states the following:
We do this only for the boot processor because the AP's can have
different base frequencies or the BIOS might boot a AP at a different
frequency.
Hope this helps.
Thanks,
Stanislav
> Michael
>
> >
> > Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@xxxxxxxxx>
> > CC: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> > CC: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > CC: Wei Liu <wei.liu@xxxxxxxxxx>
> > CC: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > CC: Ingo Molnar <mingo@xxxxxxxxxx>
> > CC: Borislav Petkov <bp@xxxxxxxxx>
> > CC: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > CC: x86@xxxxxxxxxx
> > CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > CC: linux-hyperv@xxxxxxxxxxxxxxx
> > CC: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> > arch/x86/kernel/cpu/mshyperv.c | 21 +++++++++++++++++++++
> > 1 file changed, 21 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index dedec2f23ad1..0282b2e96cc2 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -320,6 +320,21 @@ static void __init hv_smp_prepare_cpus(unsigned int
> > max_cpus)
> > }
> > #endif
> >
> > +static void __init __maybe_unused hv_preset_lpj(void)
> > +{
> > + unsigned long khz;
> > + u64 lpj;
> > +
> > + if (!x86_platform.calibrate_tsc)
> > + return;
> > +
> > + khz = x86_platform.calibrate_tsc();
> > +
> > + lpj = ((u64)khz * 1000);
> > + do_div(lpj, HZ);
> > + preset_lpj = lpj;
> > +}
> > +
> > static void __init ms_hyperv_init_platform(void)
> > {
> > int hv_max_functions_eax;
> > @@ -521,6 +536,12 @@ static void __init ms_hyperv_init_platform(void)
> >
> > /* Register Hyper-V specific clocksource */
> > hv_init_clocksource();
> > +
> > + /*
> > + * Preset lpj to make calibrate_delay a no-op, which is turn helps to
> > + * speed up secondary cores initialization.
> > + */
> > + hv_preset_lpj();
> > #endif
> > /*
> > * TSC should be marked as unstable only after Hyper-V
> >
>
