mm: batch unlink_file_vma calls in free_pgd_range

Execs of dynamically linked binaries at 20-ish cores are bottlenecked on
the i_mmap_rwsem semaphore, while the biggest singular contributor is
free_pgd_range inducing the lock acquire back-to-back for all consecutive
mappings of a given file.

Tracing the count of said acquires while building the kernel shows:
[1, 2)     799579 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[2, 3)          0 |                                                    |
[3, 4)       3009 |                                                    |
[4, 5)       3009 |                                                    |
[5, 6)     326442 |@@@@@@@@@@@@@@@@@@@@@                               |

So in particular there were 326442 opportunities to coalesce 5 acquires
into 1.

Doing so increases execs per second by 4% (~50k to ~52k) when running
the benchmark linked below.

The lock remains the main bottleneck, I have not looked at other spots
yet.

Bench can be found here:
http://apollo.backplane.com/DFlyMisc/doexec.c

$ cc -O2 -o shared-doexec doexec.c
$ ./shared-doexec $(nproc)

Note this particular test makes sure binaries are separate, but the
loader is shared.

Stats collected on the patched kernel (+ "noinline") with:
bpftrace -e 'kprobe:unlink_file_vma_batch_process
{ @ = lhist(((struct unlink_vma_file_batch *)arg0)->count, 0, 8, 1); }'

Link: https://lkml.kernel.org/r/20240521234321.359501-1-mjguzik@gmail.com
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 41 insertions, 0 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 83b4682ec85c..e42d89f98071 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -131,6 +131,47 @@ void unlink_file_vma(struct vm_area_struct *vma)
 	}
 }
 
+void unlink_file_vma_batch_init(struct unlink_vma_file_batch *vb)
+{
+	vb->count = 0;
+}
+
+static void unlink_file_vma_batch_process(struct unlink_vma_file_batch *vb)
+{
+	struct address_space *mapping;
+	int i;
+
+	mapping = vb->vmas[0]->vm_file->f_mapping;
+	i_mmap_lock_write(mapping);
+	for (i = 0; i < vb->count; i++) {
+		VM_WARN_ON_ONCE(vb->vmas[i]->vm_file->f_mapping != mapping);
+		__remove_shared_vm_struct(vb->vmas[i], mapping);
+	}
+	i_mmap_unlock_write(mapping);
+
+	unlink_file_vma_batch_init(vb);
+}
+
+void unlink_file_vma_batch_add(struct unlink_vma_file_batch *vb,
+			       struct vm_area_struct *vma)
+{
+	if (vma->vm_file == NULL)
+		return;
+
+	if ((vb->count > 0 && vb->vmas[0]->vm_file != vma->vm_file) ||
+	    vb->count == ARRAY_SIZE(vb->vmas))
+		unlink_file_vma_batch_process(vb);
+
+	vb->vmas[vb->count] = vma;
+	vb->count++;
+}
+
+void unlink_file_vma_batch_final(struct unlink_vma_file_batch *vb)
+{
+	if (vb->count > 0)
+		unlink_file_vma_batch_process(vb);
+}
+
 /*
  * Close a vm structure and free it.
  */