Re: [PATCH v2 11/12] ext4: Add allocation criteria 1.5 (CR1_5)

[Ojaswin Mujoo]

On Thu, Jun 08, 2023 at 10:45:05AM -0400, Theodore Ts'o wrote:
> Jan, thanks for the comments to Ojaswin's patch series.  Since I had
> already landed his patch series in my tree and have been testing it,
> I've fixed the obvious issues you've raised in a fixup patch
> (attached).
> 
> There is one issue which I have not fixed:
> 
> On Wed, Jun 07, 2023 at 12:21:03PM +0200, Jan Kara wrote:
> > > +	for (i = order; i >= min_order; i--) {
> > > +		int frag_order;
> > > +		/*
> > > +		 * Scale down goal len to make sure we find something
> > > +		 * in the free fragments list. Basically, reduce
> > > +		 * preallocations.
> > > +		 */
> > > +		ac->ac_g_ex.fe_len = 1 << i;
> > 
> > I smell some off-by-one issues here. Look fls(1) == 1 so (1 << fls(n)) > n.
> > Hence this loop will actually *grow* the goal allocation length. Also I'm
> > not sure why you have +1 in min_order = fls(ac->ac_o_ex.fe_len) + 1.
> 
> Ojaswin, could you take a look this?  Thanks!!
> 
> 	       	   	       	      - Ted
> 
> commit 182d2d90a180838789ed5a19e08c333043d1617a
> Author: Theodore Ts'o <tytso@xxxxxxx>
> Date:   Thu Jun 8 10:39:35 2023 -0400
> 
>     ext4: clean up mballoc criteria comments
>     
>     Line wrap and slightly clarify the comments describing mballoc's
>     cirtiera.
>     
>     Define EXT4_MB_NUM_CRS as part of the enum, so that it will
>     automatically get updated when criteria is added or removed.
>     
>     Also fix a potential unitialized use of 'cr' variable if
>     CONFIG_EXT4_DEBUG is enabled.
>     
>     Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>

Hi Ted, 

Patch looks good, thanks for doing this. I've sent the fix
for the off by one issue here:

https://lore.kernel.org/linux-ext4/20230609103403.112807-1-ojaswin@xxxxxxxxxxxxx/T/#u

Jan, thanks for the review. I've addressed the bug for now. Since
I'm on vacation for the next one and a half week I might not be able to
address the other cleanups. I'll get them done once I'm back.

Regards,
ojaswin
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 6a1f013d23f7..45a531446ea2 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -128,47 +128,52 @@ enum SHIFT_DIRECTION {
>  };
>  
>  /*
> - * Number of criterias defined. For each criteria, mballoc has slightly
> - * different way of finding the required blocks nad usually, higher the
> - * criteria the slower the allocation. We start at lower criterias and keep
> - * falling back to higher ones if we are not able to find any blocks.
> - */
> -#define EXT4_MB_NUM_CRS 5
> -/*
> - * All possible allocation criterias for mballoc. Lower are faster.
> + * For each criteria, mballoc has slightly different way of finding
> + * the required blocks nad usually, higher the criteria the slower the
> + * allocation.  We start at lower criterias and keep falling back to
> + * higher ones if we are not able to find any blocks.  Lower (earlier)
> + * criteria are faster.
>   */
>  enum criteria {
>  	/*
> -	 * Used when number of blocks needed is a power of 2. This doesn't
> -	 * trigger any disk IO except prefetch and is the fastest criteria.
> +	 * Used when number of blocks needed is a power of 2. This
> +	 * doesn't trigger any disk IO except prefetch and is the
> +	 * fastest criteria.
>  	 */
>  	CR_POWER2_ALIGNED,
>  
>  	/*
> -	 * Tries to lookup in-memory data structures to find the most suitable
> -	 * group that satisfies goal request. No disk IO except block prefetch.
> +	 * Tries to lookup in-memory data structures to find the most
> +	 * suitable group that satisfies goal request. No disk IO
> +	 * except block prefetch.
>  	 */
>  	CR_GOAL_LEN_FAST,
>  
>          /*
> -	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal length to
> -         * the best available length for faster allocation.
> +	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal
> +         * length to the best available length for faster allocation.
>  	 */
>  	CR_BEST_AVAIL_LEN,
>  
>  	/*
> -	 * Reads each block group sequentially, performing disk IO if necessary, to
> -	 * find find_suitable block group. Tries to allocate goal length but might trim
> -	 * the request if nothing is found after enough tries.
> +	 * Reads each block group sequentially, performing disk IO if
> +	 * necessary, to find find_suitable block group. Tries to
> +	 * allocate goal length but might trim the request if nothing
> +	 * is found after enough tries.
>  	 */
>  	CR_GOAL_LEN_SLOW,
>  
>  	/*
> -	 * Finds the first free set of blocks and allocates those. This is only
> -	 * used in rare cases when CR_GOAL_LEN_SLOW also fails to allocate
> -	 * anything.
> +	 * Finds the first free set of blocks and allocates
> +	 * those. This is only used in rare cases when
> +	 * CR_GOAL_LEN_SLOW also fails to allocate anything.
>  	 */
>  	CR_ANY_FREE,
> +
> +	/*
> +	 * Number of criterias defined.
> +	 */
> +	EXT4_MB_NUM_CRS
>  };
>  
>  /* criteria below which we use fast block scanning and avoid unnecessary IO */
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 8a6896d4e9b0..2f9f5dc720cc 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2759,7 +2759,7 @@ static noinline_for_stack int
>  ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  {
>  	ext4_group_t prefetch_grp = 0, ngroups, group, i;
> -	enum criteria cr, new_cr;
> +	enum criteria new_cr, cr = CR_GOAL_LEN_FAST;
>  	int err = 0, first_err = 0;
>  	unsigned int nr = 0, prefetch_ios = 0;
>  	struct ext4_sb_info *sbi;
> @@ -2816,12 +2816,13 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  		spin_unlock(&sbi->s_md_lock);
>  	}
>  
> -	/* Let's just scan groups to find more-less suitable blocks */
> -	cr = ac->ac_2order ? CR_POWER2_ALIGNED : CR_GOAL_LEN_FAST;
>  	/*
> -	 * cr == CR_POWER2_ALIGNED try to get exact allocation,
> -	 * cr == CR_ANY_FREE try to get anything
> +	 * Let's just scan groups to find more-less suitable blocks We
> +	 * start with CR_GOAL_LEN_FAST, unless it is power of 2
> +	 * aligned, in which case let's do that faster approach first.
>  	 */
> +	if (ac->ac_2order)
> +		cr = CR_POWER2_ALIGNED;
>  repeat:
>  	for (; cr < EXT4_MB_NUM_CRS && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
>  		ac->ac_criteria = cr;