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

[Vlastimil Babka]

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

> +		goto retry;
> +	}
> +
> +	list_add(&range.list, &accepting_list);
> +
>  	for_each_set_bitrange_from(range_start, range_end, unaccepted->bitmap,
>  				   DIV_ROUND_UP(end, unit_size)) {
>  		unsigned long phys_start, phys_end;
> @@ -89,9 +116,15 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
>  		phys_start = range_start * unit_size + unaccepted->phys_base;
>  		phys_end = range_end * unit_size + unaccepted->phys_base;
>  
> +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> +
>  		arch_accept_memory(phys_start, phys_end);
> +
> +		spin_lock_irqsave(&unaccepted_memory_lock, flags);
>  		bitmap_clear(unaccepted->bitmap, range_start, len);
>  	}
> +
> +	list_del(&range.list);
>  	spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
>  }
>