Re: [PATCHv2] efi/unaccepted: Fix soft lockups caused by parallel memory acceptance

[Michael Roth <michael.roth@amd.com>]

On Mon, Oct 16, 2023 at 07:31:22PM +0300, Kirill A. Shutemov wrote:
> Michael reported soft lockups on a system that has unaccepted memory.
> This occurs when a user attempts to allocate and accept memory on
> multiple CPUs simultaneously.
> 
> The root cause of the issue is that memory acceptance is serialized with
> a spinlock, allowing only one CPU to accept memory at a time. The other
> CPUs spin and wait for their turn, leading to starvation and soft lockup
> reports.
> 
> To address this, the code has been modified to release the spinlock
> while accepting memory. This allows for parallel memory acceptance on
> multiple CPUs.
> 
> A newly introduced "accepting_list" keeps track of which memory is
> currently being accepted. This is necessary to prevent parallel
> acceptance of the same memory block. If a collision occurs, the lock is
> released and the process is retried.
> 
> Such collisions should rarely occur. The main path for memory acceptance
> is the page allocator, which accepts memory in MAX_ORDER chunks. As long
> as MAX_ORDER is equal to or larger than the unit_size, collisions will
> never occur because the caller fully owns the memory block being
> accepted.
> 
> Aside from the page allocator, only memblock and deferered_free_range()
> accept memory, but this only happens during boot.
> 
> The code has been tested with unit_size == 128MiB to trigger collisions
> and validate the retry codepath.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Michael Roth <michael.roth@amd.com

Tested-by: Michael Roth <michael.roth@amd.com>

This seems to improve things pretty dramatically for me. Previously I
saw soft-lockups with 16 vCPUs and 16 processes faulting into memory,
and now I can do 128+ vCPUs/processes.

I can still trigger soft lock-ups on occassion if the number of processes
faulting in memory exceeds the number of vCPUs available to the guest, but
with a 32 vCPU guest even something like this:

  stress --vm 128 --vm-bytes 2G --vm-keep --cpu 255

still seems to avoid the soft lock-up messages. So that's probably well
into "potential future optimization" territory and this patch fixes the
more immediate issues.

Thanks!

-Mike

> Fixes: 2053bc57f367 ("efi: Add unaccepted memory support")
> Cc: <stable@kernel.org>
> Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
> ---
> 
>   v2:
>    - Fix deadlock (Vlastimil);
>    - Fix comments (Vlastimil);
>    - s/cond_resched()/cpu_relax()/ -- cond_resched() cannot be called
>      from atomic context;
>