Re: [PATCHv14 5/9] efi: Add unaccepted memory support

[Kirill A. Shutemov]

On Fri, Oct 13, 2023 at 06:44:45PM +0200, Vlastimil Babka wrote:
> On 10/13/23 18:22, Kirill A. Shutemov wrote:
> > On Fri, Oct 13, 2023 at 03:33:58PM +0300, Kirill A. Shutemov wrote:
> >> > While testing SNP guests running today's tip/master (ef19bc9dddc3) I ran
> >> > into what seems to be fairly significant lock contention due to the
> >> > unaccepted_memory_lock spinlock above, which results in a constant stream
> >> > of soft-lockups until the workload gets all its memory accepted/faulted
> >> > in if the guest has around 16+ vCPUs.
> >> > 
> >> > I've included the guest dmesg traces I was seeing below.
> >> > 
> >> > In this case I was running a 32 vCPU guest with 200GB of memory running on
> >> > a 256 thread EPYC (Milan) system, and can trigger the above situation fairly
> >> > reliably by running the following workload in a freshly-booted guests:
> >> > 
> >> >   stress --vm 32 --vm-bytes 5G --vm-keep
> >> > 
> >> > Scaling up the number of stress threads and vCPUs should make it easier
> >> > to reproduce.
> >> > 
> >> > Other than unresponsiveness/lockup messages until the memory is accepted,
> >> > the guest seems to continue running fine, but for large guests where
> >> > unaccepted memory is more likely to be useful, it seems like it could be
> >> > an issue, especially when consider 100+ vCPU guests.
> >> 
> >> Okay, sorry for delay. It took time to reproduce it with TDX.
> >> 
> >> I will look what can be done.
> > 
> > Could you check if the patch below helps?
> > 
> > diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> > index 853f7dc3c21d..591da3f368fa 100644
> > --- a/drivers/firmware/efi/unaccepted_memory.c
> > +++ b/drivers/firmware/efi/unaccepted_memory.c
> > @@ -8,6 +8,14 @@
> >  /* Protects unaccepted memory bitmap */
> >  static DEFINE_SPINLOCK(unaccepted_memory_lock);
> >  
> > +struct accept_range {
> > +	struct list_head list;
> > +	unsigned long start;
> > +	unsigned long end;
> > +};
> > +
> > +static LIST_HEAD(accepting_list);
> > +
> >  /*
> >   * accept_memory() -- Consult bitmap and accept the memory if needed.
> >   *
> > @@ -24,6 +32,7 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >  {
> >  	struct efi_unaccepted_memory *unaccepted;
> >  	unsigned long range_start, range_end;
> > +	struct accept_range range, *entry;
> >  	unsigned long flags;
> >  	u64 unit_size;
> >  
> > @@ -80,7 +89,25 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >  
> >  	range_start = start / unit_size;
> >  
> > +	range.start = start;
> > +	range.end = end;
> > +retry:
> >  	spin_lock_irqsave(&unaccepted_memory_lock, flags);
> > +
> > +	list_for_each_entry(entry, &accepting_list, list) {
> > +		if (entry->end < start)
> > +			continue;
> > +		if (entry->start > end)
> > +			continue;
> > +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> > +
> > +		/* Somebody else accepting the range */
> > +		cpu_relax();
> 
> Should this be rather cond_resched()? I think cpu_relax() isn't enough to
> prevent soft lockups.

Right. For some reason, I thought we cannot call cond_resched() from
atomic context (we sometimes get there from atomic context), but we can.

> Although IIUC hitting this should be rare, as the contending tasks will pick
> different ranges via try_to_accept_memory_one(), right?

Yes, it should be rare.

Generally, with exception of memblock, we accept all memory with MAX_ORDER
chunks. As long as unit_size <= MAX_ORDER page allocator should never
trigger the conflict as the caller owns full range to accept.

I will test the idea with larger unit_size to see how it behaves.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov