Re: [PATCH 5/6] xfs: don't try redundant allocations in xfs_rtallocate_extent_near()

[Omar Sandoval <osandov@osandov.com>]

On Mon, Jul 17, 2023 at 02:06:36PM -0700, Omar Sandoval wrote:
> On Wed, Jul 12, 2023 at 04:34:03PM -0700, Darrick J. Wong wrote:
> > On Tue, Jun 20, 2023 at 02:32:15PM -0700, Omar Sandoval wrote:
> > > From: Omar Sandoval <osandov@fb.com>
> > > 
> > > xfs_rtallocate_extent_near() tries to find a free extent as close to a
> > > target bitmap block given by bbno as possible, which may be before or
> > > after bbno. Searching backwards has a complication: the realtime summary
> > > accounts for free space _starting_ in a bitmap block, but not straddling
> > > or ending in a bitmap block. So, when the negative search finds a free
> > > extent in the realtime summary, in order to end up closer to the target,
> > > it looks for the end of the free extent. For example, if bbno - 2 has a
> > > free extent, then it will check bbno - 1, then bbno - 2. But then if
> > > bbno - 3 has a free extent, it will check bbno - 1 again, then bbno - 2
> > > again, and then bbno - 3. This results in a quadratic loop, which is
> > > completely pointless since the repeated checks won't find anything new.
> > > 
> > > Fix it by remembering where we last checked up to and continue from
> > > there. This also obviates the need for a check of the realtime summary.
> > > 
> > > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > > ---
> > >  fs/xfs/xfs_rtalloc.c | 46 +++-----------------------------------------
> > >  1 file changed, 3 insertions(+), 43 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
> > > index d079dfb77c73..4d9d0be2e616 100644
> > > --- a/fs/xfs/xfs_rtalloc.c
> > > +++ b/fs/xfs/xfs_rtalloc.c
> > > @@ -468,6 +468,7 @@ xfs_rtallocate_extent_near(
> > >  	}
> > >  	bbno = XFS_BITTOBLOCK(mp, bno);
> > >  	i = 0;
> > > +	j = -1;
> > >  	ASSERT(minlen != 0);
> > >  	log2len = xfs_highbit32(minlen);
> > >  	/*
> > > @@ -518,31 +519,11 @@ xfs_rtallocate_extent_near(
> > >  			else {		/* i < 0 */
> > >  				/*
> > >  				 * Loop backwards through the bitmap blocks from
> > > -				 * the starting point-1 up to where we are now.
> > > +				 * where we last checked up to where we are now.
> > 
> > I find this comment a bit unclear -- aren't we looping backwards from
> > where we last checked *downwards*?  I was reading "where we are now" to
> > mean @i, which contains a negative value.
> 
> Yes, "where we last checked down to where we are now" might be better
> wording.
> 
> > "When @i is negative, we try to find a free extent that starts in the
> > bitmap blocks before bbno.  Starting from the last bitmap block that we
> > checked in a negative scan (initially bbno - 1) and walking downwards
> > towards (bbno + i), try to allocate an extent of satisfactory length."
> > 
> > But now having worked my way through that, now I'm wondering what the j
> > loop is even doing.  Doesn't the sequence of blocks that we call
> > xfs_rtallocate_extent_block on alternate backwards and forwards?  e.g.
> > 
> > Try to find a satisfactory free extent that starts in:
> > 
> > bbno
> > bbno + 1
> > bbno - 1
> > bbno + 2
> > bbno - 2
> > ...
> > etc?
> > 
> > Why not avoid the loop entirely by calling xfs_rtallocate_extent_block
> > on bbno + i once before switching back to positive @i?  What am I
> > missing here?
> 
> There are two ways I can think of to remove the j loop, so I'll address
> both.
> 
> If you mean: make the i >= 0 and i < 0 branches the same and call
> xfs_rtallocate_extent_block() if and only if xfs_rtany_summary() returns
> a non-zero maxlog, i.e.:
> 
> diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
> index 4ab03eafd39f..9d7296c40ddd 100644
> --- a/fs/xfs/xfs_rtalloc.c
> +++ b/fs/xfs/xfs_rtalloc.c
> @@ -495,10 +495,6 @@ xfs_rtallocate_extent_near(
>  			xfs_extlen_t maxavail =
>  				min_t(xfs_rtblock_t, maxlen,
>  				      (1ULL << (maxlog + 1)) - 1);
> -			/*
> -			 * On the positive side of the starting location.
> -			 */
> -			if (i >= 0) {
>  			/*
>  			 * Try to allocate an extent starting in
>  			 * this block.
> @@ -517,33 +513,6 @@ xfs_rtallocate_extent_near(
>  				return 0;
>  			}
>  		}
> -			/*
> -			 * On the negative side of the starting location.
> -			 */
> -			else {		/* i < 0 */
> -				/*
> -				 * Loop backwards through the bitmap blocks from
> -				 * where we last checked up to where we are now.
> -				 * There should be an extent which ends in this
> -				 * bitmap block and is long enough.
> -				 */
> -				for (; j >= i; j--) {
> -					error = xfs_rtallocate_extent_block(mp,
> -						tp, bbno + j, minlen, maxavail,
> -						len, &n, rtbufc, prod, &r);
> -					if (error) {
> -						return error;
> -					}
> -					/*
> -					 * If it works, return the extent.
> -					 */
> -					if (r != NULLRTBLOCK) {
> -						*rtblock = r;
> -						return 0;
> -					}
> -				}
> -			}
> -		}
>  		/*
>  		 * Loop control.  If we were on the positive side, and there's
>  		 * still more blocks on the negative side, go there.
> 
> Then when i < 0, this will only find the _beginning_ of a free extent
> before bbno rather than the apparent goal of trying to allocate as close
> as possible to bbno, i.e., the _end_ of the free extent. (This is what I
> tried to explain in the commit message.)
> 
> If instead you mean: unconditionally call xfs_rtallocate_extent_block()
> for bbno + i when i < 0, i.e.:
> 
> diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
> index 4ab03eafd39f..1cf42910c0e8 100644
> --- a/fs/xfs/xfs_rtalloc.c
> +++ b/fs/xfs/xfs_rtalloc.c
> @@ -491,14 +491,10 @@ xfs_rtallocate_extent_near(
>  		 * If there are any useful extents starting here, try
>  		 * allocating one.
>  		 */
> -		if (maxlog >= 0) {
> +		if (maxlog >= 0 || i < 0) {
>  			xfs_extlen_t maxavail =
>  				min_t(xfs_rtblock_t, maxlen,
>  				      (1ULL << (maxlog + 1)) - 1);
> -			/*
> -			 * On the positive side of the starting location.
> -			 */
> -			if (i >= 0) {
>  			/*
>  			 * Try to allocate an extent starting in
>  			 * this block.
> @@ -517,33 +513,6 @@ xfs_rtallocate_extent_near(
>  				return 0;
>  			}
>  		}
> -			/*
> -			 * On the negative side of the starting location.
> -			 */
> -			else {		/* i < 0 */
> -				/*
> -				 * Loop backwards through the bitmap blocks from
> -				 * where we last checked up to where we are now.
> -				 * There should be an extent which ends in this
> -				 * bitmap block and is long enough.
> -				 */
> -				for (; j >= i; j--) {
> -					error = xfs_rtallocate_extent_block(mp,
> -						tp, bbno + j, minlen, maxavail,
> -						len, &n, rtbufc, prod, &r);
> -					if (error) {
> -						return error;
> -					}
> -					/*
> -					 * If it works, return the extent.
> -					 */
> -					if (r != NULLRTBLOCK) {
> -						*rtblock = r;
> -						return 0;
> -					}
> -				}
> -			}
> -		}
>  		/*
>  		 * Loop control.  If we were on the positive side, and there's
>  		 * still more blocks on the negative side, go there.
> 
> 
> Then this will find the end of the extent, but we will waste a lot of
> time searching bitmap blocks that don't have any usable free space. (In
> fact, this is something that patch 6 tries to reduce further.)

Ping, I hope this clarified things.