linux-kernel - Re: [PATCH] efi/unaccepted: touch soft lockup during memory accept

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Date: Wed, 10 Apr 2024 15:35:39 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: linux-efi@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Chao Gao <chao.gao@...el.com>, linux-kernel@...r.kernel.org, 
	"Hossain, Md Iqbal" <md.iqbal.hossain@...el.com>
Subject: Re: [PATCH] efi/unaccepted: touch soft lockup during memory accept

On Wed, Apr 10, 2024 at 06:23:01PM +0800, Chen Yu wrote:
> Commit 50e782a86c98 ("efi/unaccepted: Fix soft lockups caused
> by parallel memory acceptance") has released the spinlock so
> other CPUs can do memory acceptance in parallel and not
> triggers softlockup on other CPUs.
> 
> However the softlock up was intermittent shown up if the memory
> of the TD guest is large, and the timeout of softlockup is set
> to 1 second:
> 
>  RIP: 0010:_raw_spin_unlock_irqrestore
>  Call Trace:
>  ? __hrtimer_run_queues
>  <IRQ>
>  ? hrtimer_interrupt
>  ? watchdog_timer_fn
>  ? __sysvec_apic_timer_interrupt
>  ? __pfx_watchdog_timer_fn
>  ? sysvec_apic_timer_interrupt
>  </IRQ>
>  ? __hrtimer_run_queues
>  <TASK>
>  ? hrtimer_interrupt
>  ? asm_sysvec_apic_timer_interrupt
>  ? _raw_spin_unlock_irqrestore
>  ? __sysvec_apic_timer_interrupt
>  ? sysvec_apic_timer_interrupt
>  accept_memory
>  try_to_accept_memory
>  do_huge_pmd_anonymous_page
>  get_page_from_freelist
>  __handle_mm_fault
>  __alloc_pages
>  __folio_alloc
>  ? __tdx_hypercall
>  handle_mm_fault
>  vma_alloc_folio
>  do_user_addr_fault
>  do_huge_pmd_anonymous_page
>  exc_page_fault
>  ? __do_huge_pmd_anonymous_page
>  asm_exc_page_fault
>  __handle_mm_fault

Stacktrace doesn't add much value here. Just drop it.

> When the local irq is enabled at the end of accept_memory(),
> the softlockup detects that the watchdog on single CPU has
> not been fed for a while. That is to say, even other CPUs
> will not be blocked by spinlock, the current CPU might be
> stunk with local irq disabled for a while, which hurts not
> only nmi watchdog but also softlockup.
> 
> Chao Gao pointed out that the memory accept could be time
> costly and there was similar report before. Thus to avoid
> any softlocup detection during this stage, give the
> softlockup a flag to skip the timeout check at the end of
> accept_memory(), by invoking touch_softlockup_watchdog().
> 
> Reported-by: "Hossain, Md Iqbal" <md.iqbal.hossain@...el.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

And I think it is justified to add Fixes tag:

Fixes: 50e782a86c98 ("efi/unaccepted: Fix soft lockups caused by parallel memory acceptance")

-- 
  Kiryl Shutsemau / Kirill A. Shutemov