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Wong" To: Omar Sandoval Cc: linux-xfs@vger.kernel.org, kernel-team@fb.com, Prashant Nema Subject: Re: [PATCH 5/6] xfs: don't try redundant allocations in xfs_rtallocate_extent_near() Message-ID: <20230801230010.GN11352@frogsfrogsfrogs> References: <20230712233403.GY108251@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org 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 > > > > > > 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 > > > --- > > > 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.) Hmm. It's hard to remember what I was thinking 15 minutes ago let alone 2 weeks ago, but I /think/ this is what I was driving at. I agree with your statement that this would find any free extent starting before bbno, instead of a free extent *ending* as close as possible to bbno. I now understand what this patch is trying to accomplish, and it looks good to me, modulo whatever comment changes you want to make. :) > 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.) > > > * There should be an extent which ends in this > > > * bitmap block and is long enough. > > > */ > > > - for (j = -1; j > i; j--) { > > > - /* > > > - * Grab the summary information for > > > - * this bitmap block. > > > - */ > > > - error = xfs_rtany_summary(mp, tp, > > > - log2len, mp->m_rsumlevels - 1, > > > - bbno + j, rtbufc, &maxlog); > > > - if (error) { > > > - return error; > > > - } > > > - /* > > > - * If there's no extent given in the > > > - * summary that means the extent we > > > - * found must carry over from an > > > - * earlier block. If there is an > > > - * extent given, we've already tried > > > - * that allocation, don't do it again. > > > - */ > > > - if (maxlog >= 0) > > > - continue; > > > + for (; j >= i; j--) { > > > > Changing the j > i to j >= i is what obviates the extra call to > > xfs_rtallocate_extent_block below, correct? > > Correct. Before, the loop body was different because it contained a call > to xfs_rtany_summary(). But now there's no check, so the extra call can > be included in the loop. Ok, thanks. That makes sense to me now. --D