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

[Kirill A. Shutemov]

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();
+		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);
 }
 
-- 
  Kiryl Shutsemau / Kirill A. Shutemov