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
