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, 9 insertions, 0 deletions

diff --git a/mm/internal.h b/mm/internal.h
index 6902b7dd8509..774bf23b030c 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1515,4 +1515,13 @@ static inline void shrinker_debugfs_remove(struct dentry *debugfs_entry,
 void workingset_update_node(struct xa_node *node);
 extern struct list_lru shadow_nodes;
 
+struct unlink_vma_file_batch {
+	int count;
+	struct vm_area_struct *vmas[8];
+};
+
+void unlink_file_vma_batch_init(struct unlink_vma_file_batch *);
+void unlink_file_vma_batch_add(struct unlink_vma_file_batch *, struct vm_area_struct *);
+void unlink_file_vma_batch_final(struct unlink_vma_file_batch *);
+
 #endif	/* __MM_INTERNAL_H */