Re: [RFC] improve malloc for large filesystems

[Alex Zhuravlev <azhuravlev@whamcloud.com>]

> On 20 Nov 2019, at 21:13, Theodore Y. Ts'o <tytso@mit.edu> wrote:
> 
> Hi Alex,
> 
> A couple of comments.  First, please separate this patch so that these
> two separate pieces of functionality can be reviewed and tested
> separately:
> 

This is the first patch of the series.

Thanks, Alex

From 81c4b3b5a17d94525bbc6d2d89b20f6618b05bc6 Mon Sep 17 00:00:00 2001
From: Alex Zhuravlev <bzzz@whamcloud.com>
Date: Thu, 21 Nov 2019 09:53:13 +0300
Subject: [PATCH 1/2] ext4: limit scanning for a good group

at first two rounds to prevent situation when 10x-100x thousand
of groups are scanned, especially non-initialized groups.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
---
 fs/ext4/ext4.h    |  2 ++
 fs/ext4/mballoc.c | 14 ++++++++++++--
 fs/ext4/sysfs.c   |  4 ++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 03db3e71676c..d4e47fdad87c 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1480,6 +1480,8 @@ struct ext4_sb_info {
 	/* where last allocation was done - for stream allocation */
 	unsigned long s_mb_last_group;
 	unsigned long s_mb_last_start;
+	unsigned int s_mb_toscan0;
+	unsigned int s_mb_toscan1;
 
 	/* stats for buddy allocator */
 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a3e2767bdf2f..cebd7d8df0b8 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2098,7 +2098,7 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
 static noinline_for_stack int
 ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 {
-	ext4_group_t ngroups, group, i;
+	ext4_group_t ngroups, toscan, group, i;
 	int cr;
 	int err = 0, first_err = 0;
 	struct ext4_sb_info *sbi;
@@ -2169,7 +2169,15 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		 */
 		group = ac->ac_g_ex.fe_group;
 
-		for (i = 0; i < ngroups; group++, i++) {
+		/* limit number of groups to scan at the first two rounds
+		 * when we hope to find something really good */
+		toscan = ngroups;
+		if (cr == 0)
+			toscan = sbi->s_mb_toscan0;
+		else if (cr == 1)
+			toscan = sbi->s_mb_toscan1;
+
+		for (i = 0; i < toscan; group++, i++) {
 			int ret = 0;
 			cond_resched();
 			/*
@@ -2872,6 +2880,8 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
 			bio_put(discard_bio);
 		}
 	}
+	sbi->s_mb_toscan0 = 1024;
+	sbi->s_mb_toscan1 = 4096;
 
 	list_for_each_entry_safe(entry, tmp, &freed_data_list, efd_list)
 		ext4_free_data_in_buddy(sb, entry);
diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index eb1efad0e20a..c96ee20f5487 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -198,6 +198,8 @@ EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
 EXT4_ATTR(first_error_time, 0444, first_error_time);
 EXT4_ATTR(last_error_time, 0444, last_error_time);
 EXT4_ATTR(journal_task, 0444, journal_task);
+EXT4_RW_ATTR_SBI_UI(mb_toscan0, s_mb_toscan0);
+EXT4_RW_ATTR_SBI_UI(mb_toscan1, s_mb_toscan1);
 
 static unsigned int old_bump_val = 128;
 EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
@@ -228,6 +230,8 @@ static struct attribute *ext4_attrs[] = {
 	ATTR_LIST(first_error_time),
 	ATTR_LIST(last_error_time),
 	ATTR_LIST(journal_task),
+	ATTR_LIST(mb_toscan0),
+	ATTR_LIST(mb_toscan1),
 	NULL,
 };
 ATTRIBUTE_GROUPS(ext4);
-- 
2.20.1