| Message ID | 089b38635884e95cfd858c003630bdc8bd7f22b0.1642953021.git.riteshh@linux.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | jbd2: Kill age-old t_handle_lock transaction spinlock | expand |
On Sun 23-01-22 22:53:27, Ritesh Harjani wrote: > This patch kills t_handle_lock transaction spinlock completely from > jbd2. > To explain the reasoning, currently there were three sites at which this > spinlock was used. > 1. jbd2_journal_wait_updates() > a. Based on careful code review it can be seen that, we don't need this > lock here. This is since we wait for any currently ongoing updates > based on a atomic variable t_updates. And we anyway don't take any > t_handle_lock while in stop_this_handle(). > i.e. > > write_lock(&journal->j_state_lock() > jbd2_journal_wait_updates() stop_this_handle() > while (atomic_read(txn->t_updates) { | > DEFINE_WAIT(wait); | > prepare_to_wait(); | > if (atomic_read(txn->t_updates) if (atomic_dec_and_test(txn->t_updates)) > write_unlock(&journal->j_state_lock); > schedule(); wake_up() > write_lock(&journal->j_state_lock); > finish_wait(); > } > txn->t_state = T_COMMIT > write_unlock(&journal->j_state_lock); > > b. Also note that between atomic_inc(&txn->t_updates) in > start_this_handle() and jbd2_journal_wait_updates(), the > synchronization happens via read_lock(journal->j_state_lock) in > start_this_handle(); > > 2. jbd2_journal_extend() > a. jbd2_journal_extend() is called with the handle of each process from > task_struct. So no lock required in updating member fields of handle_t > > b. For member fields of h_transaction, all updates happens only via > atomic APIs (which is also within read_lock()). > So, no need of this transaction spinlock. > > 3. update_t_max_wait() > Based on Jan suggestion, this can be carefully removed using atomic > cmpxchg API. > Note that there can be several processes which are waiting for a new > transaction to be allocated and started. For doing this only one > process will succeed in taking write_lock() and allocating a new txn. > After that all of the process will be updating the t_max_wait (max > transaction wait time). This can be done via below method w/o taking > any locks using atomic cmpxchg. > For more details refer [1] > > new = get_new_val(); > old = READ_ONCE(ptr->max_val); > while (old < new) > old = cmpxchg(&ptr->max_val, old, new); > > [1]: https://lwn.net/Articles/849237/ > Suggested-by: Jan Kara <jack@suse.cz> > Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com> Looks good. Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > fs/jbd2/transaction.c | 29 +++++++++-------------------- > include/linux/jbd2.h | 3 --- > 2 files changed, 9 insertions(+), 23 deletions(-) > > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c > index 8e2f8275a253..68dd7de49aff 100644 > --- a/fs/jbd2/transaction.c > +++ b/fs/jbd2/transaction.c > @@ -107,7 +107,6 @@ static void jbd2_get_transaction(journal_t *journal, > transaction->t_start_time = ktime_get(); > transaction->t_tid = journal->j_transaction_sequence++; > transaction->t_expires = jiffies + journal->j_commit_interval; > - spin_lock_init(&transaction->t_handle_lock); > atomic_set(&transaction->t_updates, 0); > atomic_set(&transaction->t_outstanding_credits, > jbd2_descriptor_blocks_per_trans(journal) + > @@ -139,24 +138,21 @@ static void jbd2_get_transaction(journal_t *journal, > /* > * Update transaction's maximum wait time, if debugging is enabled. > * > - * In order for t_max_wait to be reliable, it must be protected by a > - * lock. But doing so will mean that start_this_handle() can not be > - * run in parallel on SMP systems, which limits our scalability. So > - * unless debugging is enabled, we no longer update t_max_wait, which > - * means that maximum wait time reported by the jbd2_run_stats > - * tracepoint will always be zero. > + * t_max_wait is carefully updated here with use of atomic compare exchange. > + * Note that there could be multiplre threads trying to do this simultaneously > + * hence using cmpxchg to avoid any use of locks in this case. > */ > static inline void update_t_max_wait(transaction_t *transaction, > unsigned long ts) > { > #ifdef CONFIG_JBD2_DEBUG > + unsigned long oldts, newts; > if (jbd2_journal_enable_debug && > time_after(transaction->t_start, ts)) { > - ts = jbd2_time_diff(ts, transaction->t_start); > - spin_lock(&transaction->t_handle_lock); > - if (ts > transaction->t_max_wait) > - transaction->t_max_wait = ts; > - spin_unlock(&transaction->t_handle_lock); > + newts = jbd2_time_diff(ts, transaction->t_start); > + oldts = READ_ONCE(transaction->t_max_wait); > + while (oldts < newts) > + oldts = cmpxchg(&transaction->t_max_wait, oldts, newts); > } > #endif > } > @@ -690,7 +686,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) > DIV_ROUND_UP( > handle->h_revoke_credits_requested, > journal->j_revoke_records_per_block); > - spin_lock(&transaction->t_handle_lock); > wanted = atomic_add_return(nblocks, > &transaction->t_outstanding_credits); > > @@ -698,7 +693,7 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) > jbd_debug(3, "denied handle %p %d blocks: " > "transaction too large\n", handle, nblocks); > atomic_sub(nblocks, &transaction->t_outstanding_credits); > - goto unlock; > + goto error_out; > } > > trace_jbd2_handle_extend(journal->j_fs_dev->bd_dev, > @@ -714,8 +709,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) > result = 0; > > jbd_debug(3, "extended handle %p by %d\n", handle, nblocks); > -unlock: > - spin_unlock(&transaction->t_handle_lock); > error_out: > read_unlock(&journal->j_state_lock); > return result; > @@ -847,22 +840,18 @@ void jbd2_journal_wait_updates(journal_t *journal) > if (!commit_transaction) > return; > > - spin_lock(&commit_transaction->t_handle_lock); > while (atomic_read(&commit_transaction->t_updates)) { > DEFINE_WAIT(wait); > > prepare_to_wait(&journal->j_wait_updates, &wait, > TASK_UNINTERRUPTIBLE); > if (atomic_read(&commit_transaction->t_updates)) { > - spin_unlock(&commit_transaction->t_handle_lock); > write_unlock(&journal->j_state_lock); > schedule(); > write_lock(&journal->j_state_lock); > - spin_lock(&commit_transaction->t_handle_lock); > } > finish_wait(&journal->j_wait_updates, &wait); > } > - spin_unlock(&commit_transaction->t_handle_lock); > } > > /** > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h > index 575c3057a98a..500a95f8c914 100644 > --- a/include/linux/jbd2.h > +++ b/include/linux/jbd2.h > @@ -554,9 +554,6 @@ struct transaction_chp_stats_s { > * ->j_list_lock > * > * j_state_lock > - * ->t_handle_lock > - * > - * j_state_lock > * ->j_list_lock (journal_unmap_buffer) > * > */ > -- > 2.31.1 >
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c index 8e2f8275a253..68dd7de49aff 100644 --- a/fs/jbd2/transaction.c +++ b/fs/jbd2/transaction.c @@ -107,7 +107,6 @@ static void jbd2_get_transaction(journal_t *journal, transaction->t_start_time = ktime_get(); transaction->t_tid = journal->j_transaction_sequence++; transaction->t_expires = jiffies + journal->j_commit_interval; - spin_lock_init(&transaction->t_handle_lock); atomic_set(&transaction->t_updates, 0); atomic_set(&transaction->t_outstanding_credits, jbd2_descriptor_blocks_per_trans(journal) + @@ -139,24 +138,21 @@ static void jbd2_get_transaction(journal_t *journal, /* * Update transaction's maximum wait time, if debugging is enabled. * - * In order for t_max_wait to be reliable, it must be protected by a - * lock. But doing so will mean that start_this_handle() can not be - * run in parallel on SMP systems, which limits our scalability. So - * unless debugging is enabled, we no longer update t_max_wait, which - * means that maximum wait time reported by the jbd2_run_stats - * tracepoint will always be zero. + * t_max_wait is carefully updated here with use of atomic compare exchange. + * Note that there could be multiplre threads trying to do this simultaneously + * hence using cmpxchg to avoid any use of locks in this case. */ static inline void update_t_max_wait(transaction_t *transaction, unsigned long ts) { #ifdef CONFIG_JBD2_DEBUG + unsigned long oldts, newts; if (jbd2_journal_enable_debug && time_after(transaction->t_start, ts)) { - ts = jbd2_time_diff(ts, transaction->t_start); - spin_lock(&transaction->t_handle_lock); - if (ts > transaction->t_max_wait) - transaction->t_max_wait = ts; - spin_unlock(&transaction->t_handle_lock); + newts = jbd2_time_diff(ts, transaction->t_start); + oldts = READ_ONCE(transaction->t_max_wait); + while (oldts < newts) + oldts = cmpxchg(&transaction->t_max_wait, oldts, newts); } #endif } @@ -690,7 +686,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) DIV_ROUND_UP( handle->h_revoke_credits_requested, journal->j_revoke_records_per_block); - spin_lock(&transaction->t_handle_lock); wanted = atomic_add_return(nblocks, &transaction->t_outstanding_credits); @@ -698,7 +693,7 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) jbd_debug(3, "denied handle %p %d blocks: " "transaction too large\n", handle, nblocks); atomic_sub(nblocks, &transaction->t_outstanding_credits); - goto unlock; + goto error_out; } trace_jbd2_handle_extend(journal->j_fs_dev->bd_dev, @@ -714,8 +709,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records) result = 0; jbd_debug(3, "extended handle %p by %d\n", handle, nblocks); -unlock: - spin_unlock(&transaction->t_handle_lock); error_out: read_unlock(&journal->j_state_lock); return result; @@ -847,22 +840,18 @@ void jbd2_journal_wait_updates(journal_t *journal) if (!commit_transaction) return; - spin_lock(&commit_transaction->t_handle_lock); while (atomic_read(&commit_transaction->t_updates)) { DEFINE_WAIT(wait); prepare_to_wait(&journal->j_wait_updates, &wait, TASK_UNINTERRUPTIBLE); if (atomic_read(&commit_transaction->t_updates)) { - spin_unlock(&commit_transaction->t_handle_lock); write_unlock(&journal->j_state_lock); schedule(); write_lock(&journal->j_state_lock); - spin_lock(&commit_transaction->t_handle_lock); } finish_wait(&journal->j_wait_updates, &wait); } - spin_unlock(&commit_transaction->t_handle_lock); } /** diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h index 575c3057a98a..500a95f8c914 100644 --- a/include/linux/jbd2.h +++ b/include/linux/jbd2.h @@ -554,9 +554,6 @@ struct transaction_chp_stats_s { * ->j_list_lock * * j_state_lock - * ->t_handle_lock - * - * j_state_lock * ->j_list_lock (journal_unmap_buffer) * */
This patch kills t_handle_lock transaction spinlock completely from jbd2. To explain the reasoning, currently there were three sites at which this spinlock was used. 1. jbd2_journal_wait_updates() a. Based on careful code review it can be seen that, we don't need this lock here. This is since we wait for any currently ongoing updates based on a atomic variable t_updates. And we anyway don't take any t_handle_lock while in stop_this_handle(). i.e. write_lock(&journal->j_state_lock() jbd2_journal_wait_updates() stop_this_handle() while (atomic_read(txn->t_updates) { | DEFINE_WAIT(wait); | prepare_to_wait(); | if (atomic_read(txn->t_updates) if (atomic_dec_and_test(txn->t_updates)) write_unlock(&journal->j_state_lock); schedule(); wake_up() write_lock(&journal->j_state_lock); finish_wait(); } txn->t_state = T_COMMIT write_unlock(&journal->j_state_lock); b. Also note that between atomic_inc(&txn->t_updates) in start_this_handle() and jbd2_journal_wait_updates(), the synchronization happens via read_lock(journal->j_state_lock) in start_this_handle(); 2. jbd2_journal_extend() a. jbd2_journal_extend() is called with the handle of each process from task_struct. So no lock required in updating member fields of handle_t b. For member fields of h_transaction, all updates happens only via atomic APIs (which is also within read_lock()). So, no need of this transaction spinlock. 3. update_t_max_wait() Based on Jan suggestion, this can be carefully removed using atomic cmpxchg API. Note that there can be several processes which are waiting for a new transaction to be allocated and started. For doing this only one process will succeed in taking write_lock() and allocating a new txn. After that all of the process will be updating the t_max_wait (max transaction wait time). This can be done via below method w/o taking any locks using atomic cmpxchg. For more details refer [1] new = get_new_val(); old = READ_ONCE(ptr->max_val); while (old < new) old = cmpxchg(&ptr->max_val, old, new); [1]: https://lwn.net/Articles/849237/ Suggested-by: Jan Kara <jack@suse.cz> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com> --- fs/jbd2/transaction.c | 29 +++++++++-------------------- include/linux/jbd2.h | 3 --- 2 files changed, 9 insertions(+), 23 deletions(-) -- 2.31.1