Messages in this thread Patch in this message | | | Date | Thu, 16 Feb 2023 11:41:01 -0800 | From | Stanislav Kinsburskii <> | Subject | Re: [PATCH] x86/hyperv: Pass on the lpj value from host to guest |
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On Tue, Feb 14, 2023 at 04:19:13PM +0000, Michael Kelley (LINUX) wrote: > From: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com> > > > > 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@gmail.com> > > CC: "K. Y. Srinivasan" <kys@microsoft.com> > > CC: Haiyang Zhang <haiyangz@microsoft.com> > > CC: Wei Liu <wei.liu@kernel.org> > > CC: Dexuan Cui <decui@microsoft.com> > > CC: Thomas Gleixner <tglx@linutronix.de> > > CC: Ingo Molnar <mingo@redhat.com> > > CC: Borislav Petkov <bp@alien8.de> > > CC: Dave Hansen <dave.hansen@linux.intel.com> > > CC: x86@kernel.org > > CC: "H. Peter Anvin" <hpa@zytor.com> > > CC: linux-hyperv@vger.kernel.org > > CC: linux-kernel@vger.kernel.org > > --- > > 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 > > >
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