Virtualization - Jan 2023 - [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle state

On Sun, 15 Jan 2023 22:01:34 -0800
"Srivatsa S. Bhat" <srivatsa at csail.mit.edu> wrote:
> From: "Srivatsa S. Bhat (VMware)" <srivatsa at
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 at infradead.org>
> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa at csail.mit.edu>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Ingo Molnar <mingo at redhat.com>
> Cc: Borislav Petkov <bp at alien8.de>
> Cc: Dave Hansen <dave.hansen at linux.intel.com>
> Cc: "H. Peter Anvin" <hpa at zytor.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki at intel.com>
> Cc: "Paul E. McKenney" <paulmck at kernel.org>
> Cc: Wyes Karny <wyes.karny at amd.com>
> Cc: Lewis Caroll <lewis.carroll at amd.com>
> Cc: Tom Lendacky <thomas.lendacky at amd.com>
> Cc: Alexey Makhalov <amakhalov at vmware.com>
> Cc: Juergen Gross <jgross at suse.com>
> Cc: x86 at kernel.org
> Cc: VMware PV-Drivers Reviewers <pv-drivers at vmware.com>
> Cc: virtualization at lists.linux-foundation.org
> Cc: kvm at vger.kernel.org
> Cc: xen-devel at lists.xenproject.org
> ---
> 
> v1:
https://lore.kernel.org/lkml/165843627080.142207.12667479241667142176.stgit at
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;
>

On Mon, Jan 16 2023 at 15:55, Igor Mammedov wrote:
> "Srivatsa S. Bhat" <srivatsa at csail.mit.edu> wrote:
>> 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.
Good point.
> 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)
For a pure guest side CPU unplug operation:

    guest$ echo 0 >/sys/devices/system/cpu/cpu$N/online

the hypervisor is not involved at all. The vCPU is not removed in that
case.

So to ensure that this ends up in HLT something like the below is
required.

Note, the removal of the comment after mwait_play_dead() is intentional
because the comment is completely bogus. Not having MWAIT is not a
failure. But that wants to be a seperate patch.

Thanks,

        tglx
---        
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 55cad72715d9..3f1f20f71ec5 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1833,7 +1833,10 @@ void native_play_dead(void)
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);
 
-	mwait_play_dead();	/* Only returns on failure */
+	if (this_cpu_has(X86_FEATURE_HYPERVISOR))
+		hlt_play_dead();
+
+	mwait_play_dead();
 	if (cpuidle_play_dead())
 		hlt_play_dead();
 }