Re: [PATCH] mm: reduce spinlock contention in release_pages()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 25, 2021 at 08:02:38AM +0000, Hao Lee wrote:
> On Thu, Nov 25, 2021 at 03:30:44AM +0000, Matthew Wilcox wrote:
> > On Thu, Nov 25, 2021 at 11:24:02AM +0800, Hao Lee wrote:
> > > On Thu, Nov 25, 2021 at 12:31 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > We do batch currently so no single task should be
> > > > able to monopolize the cpu for too long. Why this is not sufficient?
> > > 
> > > uncharge and unref indeed take advantage of the batch process, but
> > > del_from_lru needs more time to complete. Several tasks will contend
> > > spinlock in the loop if nr is very large.
> > 
> > Is SWAP_CLUSTER_MAX too large?  Or does your architecture's spinlock
> > implementation need to be fixed?
> > 
> 
> My testing server is x86_64 with 5.16-rc2. The spinlock should be normal.
> 
> I think lock_batch is not the point. lock_batch only break spinning time
> into small parts, but it doesn't reduce spinning time. The thing may get
> worse if lock_batch is very small.

OK.  So if I understand right, you've got a lot of processes all
calling exit_mmap() at the same time, which eventually becomes calls to
unmap_vmas(), unmap_single_vma(), unmap_page_range(), zap_pte_range(),
tlb_flush_mmu(), tlb_batch_pages_flush(), free_pages_and_swap_cache(),
release_pages(), and then you see high contention on the LRU lock.

Your use-case doesn't seem to mind sleeping (after all, these processes
are exiting).  So we could put a semaphore in exit_mmap() to limit the
number of simultaneous callers to unmap_vmas().  Do you want to try
that out and see if it solves your problem?  You might want to make it
a counting semaphore (eg permit two tasks to exit at once) rather than
a mutex.  But maybe a mutex is just fine.