Re: [RFC 1/1] ext4: Improve fast_commit performance and scalability

From: Ritesh Harjani
Date: Thu Feb 17 2022 - 10:58:10 EST

On 22/02/16 03:25PM, harshad shirwadkar wrote:
> Thanks for the patch Ritesh. Some questions / comments inlined:

Thanks a lot for reviewing this :)

>
> On Sun, 13 Feb 2022 at 19:57, Ritesh Harjani <riteshh@xxxxxxxxxxxxx> wrote:
> >
> > Currently ext4_fc_commit_dentry_updates() is of quadratic time
> > complexity, which is causing performance bottlenecks with high
> > threads/file/dir count with fs_mark.
> >
> > This patch makes commit dentry updates (and hence ext4_fc_commit()) path
> > to linear time complexity. Hence improves the performance of workloads
> > which does fsync on multiple threads/open files one-by-one.
> >
> > Absolute numbers in avg file creates per sec (from fs_mark in 1K order)
> > =======================================================================
> > no. Order without-patch(K) with-patch(K) Diff(%)
> > 1 1 16.90 17.51 +3.60
> > 2 2,2 32.08 31.80 -0.87
> > 3 3,3 53.97 55.01 +1.92
> > 4 4,4 78.94 76.90 -2.58
> > 5 5,5 95.82 95.37 -0.46
> > 6 6,6 87.92 103.38 +17.58
> > 7 6,10 0.73 126.13 +17178.08
> > 8 6,14 2.33 143.19 +6045.49
> >
> > workload type
> > ==============
> > For e.g. 7th row order of 6,10 (2^6 == 64 && 2^10 == 1024)
> > echo /run/riteshh/mnt/{1..64} |sed -E 's/[[:space:]]+/ -d /g' \
> > | xargs -I {} bash -c "sudo fs_mark -L 100 -D 1024 -n 1024 -s0 -S5 -d {}"
> >
> > Perf profile
> > (w/o patches)
> > =============================
> > 87.15% [kernel] [k] ext4_fc_commit --> Heavy contention/bottleneck
> > 1.98% [kernel] [k] perf_event_interrupt
> > 0.96% [kernel] [k] power_pmu_enable
> > 0.91% [kernel] [k] update_sd_lb_stats.constprop.0
> > 0.67% [kernel] [k] ktime_get
> >
> > Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
> > ---
> > fs/ext4/ext4.h | 2 ++
> > fs/ext4/fast_commit.c | 64 +++++++++++++++++++++++++++++++------------
> > fs/ext4/fast_commit.h | 1 +
> > 3 files changed, 50 insertions(+), 17 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index bcd3b9bf8069..25242648d8c9 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1046,6 +1046,8 @@ struct ext4_inode_info {
> >
> > /* Fast commit related info */
> >
> > + /* For tracking dentry create updates */
> > + struct list_head i_fc_dilist;
> The only case in which this list will have multiple entries if hard
> links are created on this inode right? I think that's probably a very

So I too had this thought on my mind later. But then I ended up coding the old way
only.

Ok, so it seems it is only when the first time an inode is created we
will have a EXT4_FC_TAG_CREAT. When we are creating a hard link that's actually
a EXT4_FC_TAG_LINK.
So I think there shouldn't be any case where we have more than one fc_dentry for
the same inode. Your thoughts?


> rare scenario and we can just fallback to full commits. That might
> simplify this patch a bit. Basically if you do that then fc_dentry
> would directly store a pointer to the inode and the inode can store a
> pointer to the "CREAT" fc_dentry object. That way we don't have to do
> list traversals in fc_del and fc_commit. But barring a few fixes, what
> you have here is fine too. So I'll leave it up to you to decide what
> you want to do.

Yes, you are right. If there is only a single fc_dentry object for any given
inode, then we can store back pointers in each of those to point to their
respective inode and fc_dentry objects.

I will try and change this in next revision then.

> > struct list_head i_fc_list; /*
> > * inodes that need fast commit
> > * protected by sbi->s_fc_lock.
> > diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> > index 7964ee34e322..f2bee4cf5648 100644
> > --- a/fs/ext4/fast_commit.c
> > +++ b/fs/ext4/fast_commit.c
> > @@ -199,6 +199,7 @@ void ext4_fc_init_inode(struct inode *inode)
> > ext4_fc_reset_inode(inode);
> > ext4_clear_inode_state(inode, EXT4_STATE_FC_COMMITTING);
> > INIT_LIST_HEAD(&ei->i_fc_list);
> > + INIT_LIST_HEAD(&ei->i_fc_dilist);
> > init_waitqueue_head(&ei->i_fc_wait);
> > atomic_set(&ei->i_fc_updates, 0);
> > }
> > @@ -279,6 +280,8 @@ void ext4_fc_stop_update(struct inode *inode)
> > void ext4_fc_del(struct inode *inode)
> > {
> > struct ext4_inode_info *ei = EXT4_I(inode);
> > + struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> > + struct ext4_fc_dentry_update *fc_dentry, *fc_dentry_n;
> >
> > if (!test_opt2(inode->i_sb, JOURNAL_FAST_COMMIT) ||
> > (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY))
> > @@ -286,7 +289,7 @@ void ext4_fc_del(struct inode *inode)
> >
> > restart:
> > spin_lock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > - if (list_empty(&ei->i_fc_list)) {
> > + if (list_empty(&ei->i_fc_list) && list_empty(&ei->i_fc_dilist)) {
> > spin_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > return;
> > }
> > @@ -295,7 +298,26 @@ void ext4_fc_del(struct inode *inode)
> > ext4_fc_wait_committing_inode(inode);
> > goto restart;
> > }
> > - list_del_init(&ei->i_fc_list);
> > +
> > + if (!list_empty(&ei->i_fc_list))
> > + list_del_init(&ei->i_fc_list);
> > +
> > + /*
> > + * Since this inode is getting removed, let's also remove all FC
> > + * dentry create references, since it is not needed to log it anyways.
> > + */
> > + list_for_each_entry_safe(fc_dentry, fc_dentry_n, &ei->i_fc_dilist, fcd_dilist) {
> > + WARN_ON(fc_dentry->fcd_op != EXT4_FC_TAG_CREAT);
> > + list_del_init(&fc_dentry->fcd_list);
> > + list_del_init(&fc_dentry->fcd_dilist);
> > + spin_unlock(&sbi->s_fc_lock);
> > +
> > + if (fc_dentry->fcd_name.name &&
> > + fc_dentry->fcd_name.len > DNAME_INLINE_LEN)
> > + kfree(fc_dentry->fcd_name.name);
> > + kmem_cache_free(ext4_fc_dentry_cachep, fc_dentry);
> > + return;
> Shouldn't we continue and remove all nodes in ei->i_fc_dilist?

Yes, I guess this survived, since we anyway have only one entry in the list
always. But thanks for catching.

> > + }
> > spin_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > }
> >
> > @@ -427,7 +449,7 @@ static int __track_dentry_update(struct inode *inode, void *arg, bool update)
> > node->fcd_name.name = node->fcd_iname;
> > }
> > node->fcd_name.len = dentry->d_name.len;
> > -
> > + INIT_LIST_HEAD(&node->fcd_dilist);
> > spin_lock(&sbi->s_fc_lock);
> > if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
> > sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING)
> > @@ -435,6 +457,18 @@ static int __track_dentry_update(struct inode *inode, void *arg, bool update)
> > &sbi->s_fc_dentry_q[FC_Q_STAGING]);
> > else
> > list_add_tail(&node->fcd_list, &sbi->s_fc_dentry_q[FC_Q_MAIN]);
> > +
> > + /*
> > + * This helps us keep a track of all fc_dentry updates which is part of
> > + * this ext4 inode. So in case the inode is getting unlinked, before
> > + * even we get a chance to fsync, we could remove all fc_dentry
> > + * references while evicting the inode in ext4_fc_del().
> > + * Also with this, we don't need to loop over all the inodes in
> > + * sbi->s_fc_q to get the corresponding inode in
> > + * ext4_fc_commit_dentry_updates().
> > + */
> > + if (dentry_update->op == EXT4_FC_TAG_CREAT)
> > + list_add_tail(&node->fcd_dilist, &ei->i_fc_dilist);
> > spin_unlock(&sbi->s_fc_lock);
> > mutex_lock(&ei->i_fc_lock);
> >
> > @@ -954,7 +988,7 @@ __releases(&sbi->s_fc_lock)
> > struct ext4_sb_info *sbi = EXT4_SB(sb);
> > struct ext4_fc_dentry_update *fc_dentry, *fc_dentry_n;
> > struct inode *inode;
> > - struct ext4_inode_info *ei, *ei_n;
> > + struct ext4_inode_info *ei;
> > int ret;
> >
> > if (list_empty(&sbi->s_fc_dentry_q[FC_Q_MAIN]))
> > @@ -970,21 +1004,16 @@ __releases(&sbi->s_fc_lock)
> > spin_lock(&sbi->s_fc_lock);
> > continue;
> > }
> > -
> > - inode = NULL;
> > - list_for_each_entry_safe(ei, ei_n, &sbi->s_fc_q[FC_Q_MAIN],
> > - i_fc_list) {
> > - if (ei->vfs_inode.i_ino == fc_dentry->fcd_ino) {
> > - inode = &ei->vfs_inode;
> > - break;
> > - }
> > - }
> > /*
> > - * If we don't find inode in our list, then it was deleted,
> > - * in which case, we don't need to record it's create tag.
> > + * With fcd_dilist we need not loop in sbi->s_fc_q to get the
> > + * corresponding inode pointer
> > */
> > - if (!inode)
> > - continue;
> > + WARN_ON(list_empty(&fc_dentry->fcd_dilist));
> > + ei = list_entry(fc_dentry->fcd_dilist.next,
> > + struct ext4_inode_info, i_fc_dilist);
> I think we want "fc_dentry->fcd_ilist.prev" here right? We are
> sequentially traversing all the nodes in the list from first to last.
> Given that I think the inode is the prev of any node that you
> encounter in the list.

Not that this will be relevant in the next iteration. But doesn't matter right,
next and prev both will have pointer to inode (since it is a circular doubly
linked list)? And we are talking about fcd_dilist right?

-ritesh

>
> - Harshad
> > + inode = &ei->vfs_inode;
> > + WARN_ON(inode->i_ino != fc_dentry->fcd_ino);
> > +
> > spin_unlock(&sbi->s_fc_lock);
> >
> > /*
> > @@ -1228,6 +1257,7 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
> > struct ext4_fc_dentry_update,
> > fcd_list);
> > list_del_init(&fc_dentry->fcd_list);
> > + list_del_init(&fc_dentry->fcd_dilist);
> > spin_unlock(&sbi->s_fc_lock);
> >
> > if (fc_dentry->fcd_name.name &&
> > diff --git a/fs/ext4/fast_commit.h b/fs/ext4/fast_commit.h
> > index 083ad1cb705a..02afa52e8e41 100644
> > --- a/fs/ext4/fast_commit.h
> > +++ b/fs/ext4/fast_commit.h
> > @@ -109,6 +109,7 @@ struct ext4_fc_dentry_update {
> > struct qstr fcd_name; /* Dirent name */
> > unsigned char fcd_iname[DNAME_INLINE_LEN]; /* Dirent name string */
> > struct list_head fcd_list;
> > + struct list_head fcd_dilist;
> > };
> >
> > struct ext4_fc_stats {
> > --
> > 2.31.1
> >