Re: [PATCH] ext4: improve smp scalability for inode generation

[Theodore Ts'o <tytso@mit.edu>]

Dmitry, can you try benchmarking this patch?

Thanks!!

						- Ted

commit f0e922e7235e1b5ba6fd964e2cf8dafed3248a15
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Wed Nov 8 22:21:58 2017 -0500

    ext4: improve smp scalability for inode generation
    
    ->s_next_generation is protected by s_next_gen_lock but its usage
    pattern is very primitive.  We don't actually need sequentailly
    increasing new generation numbers, so let's use prandom_u32() instead.
    
    Reported-by: Dmitry Monakhov <dmonakhov@openvz.org>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 53ce95b52fd8..5e6d7b6f50c7 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1355,8 +1355,6 @@ struct ext4_sb_info {
 	int s_first_ino;
 	unsigned int s_inode_readahead_blks;
 	unsigned int s_inode_goal;
-	spinlock_t s_next_gen_lock;
-	u32 s_next_generation;
 	u32 s_hash_seed[4];
 	int s_def_hash_version;
 	int s_hash_unsigned;	/* 3 if hash should be signed, 0 if not */
diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index ee823022aa34..da79eb5dba40 100644
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -1138,9 +1138,7 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
 			   inode->i_ino);
 		goto out;
 	}
-	spin_lock(&sbi->s_next_gen_lock);
-	inode->i_generation = sbi->s_next_generation++;
-	spin_unlock(&sbi->s_next_gen_lock);
+	inode->i_generation = prandom_u32();
 
 	/* Precompute checksum seed for inode metadata */
 	if (ext4_has_metadata_csum(sb)) {
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 144bbda2b808..98ad8172dfd3 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -17,6 +17,7 @@
 #include <linux/uuid.h>
 #include <linux/uaccess.h>
 #include <linux/delay.h>
+#include <linux/random.h>
 #include "ext4_jbd2.h"
 #include "ext4.h"
 #include <linux/fsmap.h>
@@ -157,10 +158,8 @@ static long swap_inode_boot_loader(struct super_block *sb,
 
 	inode->i_ctime = inode_bl->i_ctime = current_time(inode);
 
-	spin_lock(&sbi->s_next_gen_lock);
-	inode->i_generation = sbi->s_next_generation++;
-	inode_bl->i_generation = sbi->s_next_generation++;
-	spin_unlock(&sbi->s_next_gen_lock);
+	inode->i_generation = prandom_u32();
+	inode_bl->i_generation = prandom_u32();
 
 	ext4_discard_preallocations(inode);
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 3a278faf5868..9f2e3eb5131f 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3982,8 +3982,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	sbi->s_gdb_count = db_count;
-	get_random_bytes(&sbi->s_next_generation, sizeof(u32));
-	spin_lock_init(&sbi->s_next_gen_lock);
 
 	timer_setup(&sbi->s_err_report, print_daily_error_info, 0);