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Message-ID: <20230608144505.GA1422249@mit.edu>
Date: Thu, 8 Jun 2023 10:45:05 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: Jan Kara <jack@...e.cz>
Cc: Ojaswin Mujoo <ojaswin@...ux.ibm.com>, linux-ext4@...r.kernel.org,
Ritesh Harjani <riteshh@...ux.ibm.com>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
Kemeng Shi <shikemeng@...weicloud.com>,
Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [PATCH v2 11/12] ext4: Add allocation criteria 1.5 (CR1_5)
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@....edu>
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@....edu>
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;
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