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Message-ID: <20181127102939.GG16301@quack2.suse.cz>
Date: Tue, 27 Nov 2018 11:29:39 +0100
From: Jan Kara <jack@...e.cz>
To: Ted Tso <tytso@....edu>
Cc: linux-ext4@...r.kernel.org,
"Hunter, Adrian" <adrian.hunter@...el.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2] jbd2: Avoid long hold times of j_state_lock while
committing a transaction
On Thu 08-11-18 13:29:38, Jan Kara wrote:
> We can hold j_state_lock for writing at the beginning of
> jbd2_journal_commit_transaction() for a rather long time (reportedly for
> 30 ms) due cleaning revoke bits of all revoked buffers under it. The
> handling of revoke tables as well as cleaning of t_reserved_list, and
> checkpoint lists does not need j_state_lock for anything. It is only
> needed to prevent new handles from joining the transaction. Generally
> T_LOCKED transaction state prevents new handles from joining the
> transaction - except for reserved handles which have to allowed to join
> while we wait for other handles to complete.
>
> To prevent reserved handles from joining the transaction while cleaning
> up lists, add new transaction state T_SWITCH and watch for it when
> starting reserved handles. With this we can just drop the lock for
> operations that don't need it.
>
> Reported-and-tested-by: Adrian Hunter <adrian.hunter@...el.com>
> Suggested-by: "Theodore Y. Ts'o" <tytso@....edu>
> Signed-off-by: Jan Kara <jack@...e.cz>
Ping?
Honza
> ---
> fs/jbd2/commit.c | 3 +++
> fs/jbd2/transaction.c | 43 ++++++++++++++++++++++++++++++++++++++-----
> include/linux/jbd2.h | 1 +
> 3 files changed, 42 insertions(+), 5 deletions(-)
>
> Changes since v1:
> - fixed handling of reserved handles
> - rebased on top of 4.20-rc1
> - tested dioread-nolock xfstest run
>
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 150cc030b4d7..2eb55c3361a8 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -439,6 +439,8 @@ void jbd2_journal_commit_transaction(journal_t *journal)
> finish_wait(&journal->j_wait_updates, &wait);
> }
> spin_unlock(&commit_transaction->t_handle_lock);
> + commit_transaction->t_state = T_SWITCH;
> + write_unlock(&journal->j_state_lock);
>
> J_ASSERT (atomic_read(&commit_transaction->t_outstanding_credits) <=
> journal->j_max_transaction_buffers);
> @@ -505,6 +507,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
> atomic_sub(atomic_read(&journal->j_reserved_credits),
> &commit_transaction->t_outstanding_credits);
>
> + write_lock(&journal->j_state_lock);
> trace_jbd2_commit_flushing(journal, commit_transaction);
> stats.run.rs_flushing = jiffies;
> stats.run.rs_locked = jbd2_time_diff(stats.run.rs_locked,
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index c0b66a7a795b..116d8251fbff 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -138,9 +138,9 @@ static inline void update_t_max_wait(transaction_t *transaction,
> }
>
> /*
> - * Wait until running transaction passes T_LOCKED state. Also starts the commit
> - * if needed. The function expects running transaction to exist and releases
> - * j_state_lock.
> + * Wait until running transaction passes to T_FLUSH state and new transaction
> + * can thus be started. Also starts the commit if needed. The function expects
> + * running transaction to exist and releases j_state_lock.
> */
> static void wait_transaction_locked(journal_t *journal)
> __releases(journal->j_state_lock)
> @@ -160,6 +160,32 @@ static void wait_transaction_locked(journal_t *journal)
> finish_wait(&journal->j_wait_transaction_locked, &wait);
> }
>
> +/*
> + * Wait until running transaction transitions from T_SWITCH to T_FLUSH
> + * state and new transaction can thus be started. The function releases
> + * j_state_lock.
> + */
> +static void wait_transaction_switching(journal_t *journal)
> + __releases(journal->j_state_lock)
> +{
> + DEFINE_WAIT(wait);
> +
> + if (WARN_ON(!journal->j_running_transaction ||
> + journal->j_running_transaction->t_state != T_SWITCH))
> + return;
> + prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> + TASK_UNINTERRUPTIBLE);
> + read_unlock(&journal->j_state_lock);
> + /*
> + * We don't call jbd2_might_wait_for_commit() here as there's no
> + * waiting for outstanding handles happening anymore in T_SWITCH state
> + * and handling of reserved handles actually relies on that for
> + * correctness.
> + */
> + schedule();
> + finish_wait(&journal->j_wait_transaction_locked, &wait);
> +}
> +
> static void sub_reserved_credits(journal_t *journal, int blocks)
> {
> atomic_sub(blocks, &journal->j_reserved_credits);
> @@ -183,7 +209,8 @@ static int add_transaction_credits(journal_t *journal, int blocks,
> * If the current transaction is locked down for commit, wait
> * for the lock to be released.
> */
> - if (t->t_state == T_LOCKED) {
> + if (t->t_state != T_RUNNING) {
> + WARN_ON_ONCE(t->t_state >= T_FLUSH);
> wait_transaction_locked(journal);
> return 1;
> }
> @@ -360,8 +387,14 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
> /*
> * We have handle reserved so we are allowed to join T_LOCKED
> * transaction and we don't have to check for transaction size
> - * and journal space.
> + * and journal space. But we still have to wait while running
> + * transaction is being switched to a committing one as it
> + * won't wait for any handles anymore.
> */
> + if (transaction->t_state == T_SWITCH) {
> + wait_transaction_switching(journal);
> + goto repeat;
> + }
> sub_reserved_credits(journal, blocks);
> handle->h_reserved = 0;
> }
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index b708e5169d1d..118d00a64184 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -575,6 +575,7 @@ struct transaction_s
> enum {
> T_RUNNING,
> T_LOCKED,
> + T_SWITCH,
> T_FLUSH,
> T_COMMIT,
> T_COMMIT_DFLUSH,
> --
> 2.16.4
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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