On Fri, May 03, 2024 at 12:31:12PM +0200, Ard Biesheuvel wrote:
> On Wed, 24 Apr 2024 at 19:12, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> > On Mon, 22 Apr 2024 at 16:40, Chen Yu <yu.c.chen@xxxxxxxxx> wrote:
> > >
> > > On 2024-04-11 at 08:49:07 +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.
> > > >
> > > > The symptom is:
> > > > 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().
> > > >
> > > > Fixes: 50e782a86c98 ("efi/unaccepted: Fix soft lockups caused by parallel memory acceptance")
> > > > Reported-by: "Hossain, Md Iqbal" <md.iqbal.hossain@xxxxxxxxx>
> > > > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > > > ---
> > > > v1 -> v2:
> > > > Refine the commit log and add fixes tag/reviewed-by tag from Kirill.
> > >
> > > Gently pinging about this patch.
> > >
> >
> > Queued up in efi/urgent now, thanks.
>
> OK, I was about to send this patch to Linus (and I am still going to).
>
> However, I do wonder if sprinkling touch_softlockup_watchdog() left
> and right is really the right solution here.
>
> Looking at the backtrace, this is a page fault originating in user
> space. So why do we end up calling into the hypervisor to accept a
> chunk of memory large enough to trigger the softlockup watchdog? Or is
> the hypercall simply taking a disproportionate amount of time?
Note that softlockup timeout was set to 1 second to trigger this. So this
is exaggerated case.
> And AIUI, touch_softlockup_watchdog() hides the fact that we are
> hogging the CPU for way too long - is there any way we can at least
> yield the CPU on this condition?
Not really. There's no magic entity that handles accept. It is done by
CPU.
There's a feature in pipeline that makes page accept interruptable in TDX
guest. It can help some cases. But if ended up in this codepath from
non-preemptable context, it won't help.
--
Kiryl Shutsemau / Kirill A. Shutemov
