Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle state

[Igor Mammedov <imammedo@redhat.com>]

On Sun, 15 Jan 2023 22:01:34 -0800
"Srivatsa S. Bhat" <srivatsa@csail.mit.edu> wrote:

> From: "Srivatsa S. Bhat (VMware)" <srivatsa@csail.mit.edu>
> 
> Under hypervisors that support mwait passthrough, a vCPU in mwait
> CPU-idle state remains in guest context (instead of yielding to the
> hypervisor via VMEXIT), which helps speed up wakeups from idle.
> 
> However, this runs into problems with CPU hotplug, because the Linux
> CPU offline path prefers to put the vCPU-to-be-offlined in mwait
> state, whenever mwait is available. As a result, since a vCPU in mwait
> remains in guest context and does not yield to the hypervisor, an
> offline vCPU *appears* to be 100% busy as viewed from the host, which
> prevents the hypervisor from running other vCPUs or workloads on the
> corresponding pCPU. [ Note that such a vCPU is not actually busy
> spinning though; it remains in mwait idle state in the guest ].
>
> Fix this by preventing the use of mwait idle state in the vCPU offline
> play_dead() path for any hypervisor, even if mwait support is
> available.

if mwait is enabled, it's very likely guest to have cpuidle
enabled and using the same mwait as well. So exiting early from
 mwait_play_dead(), might just punt workflow down:
  native_play_dead()
        ...
        mwait_play_dead();
        if (cpuidle_play_dead())   <- possible mwait here                                              
                hlt_play_dead(); 

and it will end up in mwait again and only if that fails
it will go HLT route and maybe transition to VMM.

Instead of workaround on guest side,
shouldn't hypervisor force VMEXIT on being uplugged vCPU when it's
actually hot-unplugging vCPU? (ex: QEMU kicks vCPU out from guest
context when it is removing vCPU, among other things)

> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Wyes Karny <wyes.karny@amd.com>
> Cc: Lewis Caroll <lewis.carroll@amd.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Alexey Makhalov <amakhalov@vmware.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: x86@kernel.org
> Cc: VMware PV-Drivers Reviewers <pv-drivers@vmware.com>
> Cc: virtualization@lists.linux-foundation.org
> Cc: kvm@vger.kernel.org
> Cc: xen-devel@lists.xenproject.org
> ---
> 
> v1: https://lore.kernel.org/lkml/165843627080.142207.12667479241667142176.stgit@csail.mit.edu/
> 
>  arch/x86/kernel/smpboot.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 55cad72715d9..125a5d4bfded 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1763,6 +1763,15 @@ static inline void mwait_play_dead(void)
>  		return;
>  	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
>  		return;
> +
> +	/*
> +	 * Do not use mwait in CPU offline play_dead if running under
> +	 * any hypervisor, to make sure that the offline vCPU actually
> +	 * yields to the hypervisor (which may not happen otherwise if
> +	 * the hypervisor supports mwait passthrough).
> +	 */
> +	if (this_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return;
>  	if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
>  		return;
>