Re: [PATCH 2/2] jbd2: bound shrinker scans by examined checkpoint buffers

[Jan Kara]

On Mon 13-07-26 12:22:29, Max Kellermann wrote:
> The jbd2 shrinker currently accounts only checkpoint buffers that it
> successfully releases against nr_to_scan.  Busy buffers therefore do not
> consume the scan budget.
> 
> If a checkpoint transaction contains mostly busy buffers, the shrinker
> can scan its entire checkpoint list while holding journal->j_list_lock.
> Large checkpoint lists can result in excessive lock hold times and leave
> other CPUs spinning on j_list_lock, causing soft lockups or RCU stalls.
> 
> Pass nr_to_scan into journal_shrink_one_cp_list() and decrement it for
> every buffer examined, including busy buffers.  Pass NULL from checkpoint
> cleanup paths so their existing full-list behavior is preserved.
> 
> This restores the scan-budget semantics that existed before
> journal_shrink_one_cp_list() was changed to always scan a complete
> checkpoint list.
> 
> Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/jbd2/checkpoint.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 5266017565ac..513273712010 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -358,15 +358,16 @@ int jbd2_cleanup_journal_tail(journal_t *journal)
>  /*
>   * journal_shrink_one_cp_list
>   *
> - * Find all the written-back checkpoint buffers in the given list
> - * and try to release them. If the whole transaction is released, set
> - * the 'released' parameter. Return the number of released checkpointed
> - * buffers.
> + * Find written-back checkpoint buffers in the given list and try to release
> + * them. If 'nr_to_scan' is set, scan at most that many buffers. If the whole
> + * transaction is released, set the 'released' parameter. Return the number of
> + * released checkpointed buffers.
>   *
>   * Called with j_list_lock held.
>   */
>  static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  						enum jbd2_shrink_type type,
> +						unsigned long *nr_to_scan,
>  						bool *released)
>  {
>  	struct journal_head *last_jh;
> @@ -375,13 +376,15 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  	int ret;
>  
>  	*released = false;
> -	if (!jh)
> +	if (!jh || (nr_to_scan && !*nr_to_scan))
>  		return 0;
>  
>  	last_jh = jh->b_cpprev;
>  	do {
>  		jh = next_jh;
>  		next_jh = jh->b_cpnext;
> +		if (nr_to_scan)
> +			(*nr_to_scan)--;
>  
>  		if (type == JBD2_SHRINK_DESTROY) {
>  			ret = __jbd2_journal_remove_checkpoint(jh);
> @@ -403,7 +406,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  next:
>  		if (need_resched())
>  			break;
> -	} while (jh != last_jh);
> +	} while (jh != last_jh && (!nr_to_scan || *nr_to_scan));
>  
>  	return nr_freed;
>  }
> @@ -425,7 +428,6 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>  	tid_t first_tid = 0, last_tid = 0, next_tid = 0;
>  	tid_t tid = 0;
>  	unsigned long nr_freed = 0;
> -	unsigned long freed;
>  	bool first_set = false;
>  
>  again:
> @@ -458,10 +460,9 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>  		next_transaction = transaction->t_cpnext;
>  		tid = transaction->t_tid;
>  
> -		freed = journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> -						   JBD2_SHRINK_BUSY_SKIP, &released);
> -		nr_freed += freed;
> -		(*nr_to_scan) -= min(*nr_to_scan, freed);
> +		nr_freed += journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> +						       JBD2_SHRINK_BUSY_SKIP,
> +						       nr_to_scan, &released);
>  		if (*nr_to_scan == 0)
>  			break;
>  		if (need_resched() || spin_needbreak(&journal->j_list_lock))
> @@ -517,7 +518,7 @@ void __jbd2_journal_clean_checkpoint_list(journal_t *journal,
>  		transaction = next_transaction;
>  		next_transaction = transaction->t_cpnext;
>  		journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> -					   type, &released);
> +					   type, NULL, &released);
>  		/*
>  		 * This function only frees up some memory if possible so we
>  		 * dont have an obligation to finish processing. Bail out if
> -- 
> 2.47.3
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR