There are a few nits in this function. They could in theory all
be separate patches, but that's probably taking small commits
too far.
1) I added a brief comment saying what it does.
2) I like to declare pointer parameters "const" where possible
for documentation reasons.
3) It uses bitmap_weight(&rand, BITS_PER_LONG) to compute the Hamming
weight of a 32-bit random number (giving a random integer with
mean 16 and variance 8). Passing by reference in a 64-bit variable
is silly; just use hweight32().
4) Its helper function fract_exp_two is unnecessarily tangled.
Gcc can optimize the multiply by (1 << x) to a shift, but it can
be written in a much more straightforward way at the cost of one
more bit of internal precision. Some analysis reveals that this
bit is always available.
This shrinks the object code for fract_exp_two(x, 6) from 23 bytes:
0000000000000000 <foo1>:
0: 89 f9 mov %edi,%ecx
2: c1 e9 06 shr $0x6,%ecx
5: b8 01 00 00 00 mov $0x1,%eax
a: d3 e0 shl %cl,%eax
c: 83 e7 3f and $0x3f,%edi
f: d3 e7 shl %cl,%edi
11: c1 ef 06 shr $0x6,%edi
14: 01 f8 add %edi,%eax
16: c3 retq
To 19:
0000000000000017 <foo2>:
17: 89 f8 mov %edi,%eax
19: 83 e0 3f and $0x3f,%eax
1c: 83 c0 40 add $0x40,%eax
1f: 89 f9 mov %edi,%ecx
21: c1 e9 06 shr $0x6,%ecx
24: d3 e0 shl %cl,%eax
26: c1 e8 06 shr $0x6,%eax
29: c3 retq
(Verified with 0 <= frac_bits <= 8, 0 <= x < 16<<frac_bits;
both versions produce the same output.)
5) And finally, the call to bch_get_congested() in check_should_bypass()
is separated from the use of the value by multiple tests which
could moot the need to compute it. Move the computation down to
where it's needed. This also saves a local register to hold the
computed value.
Signed-off-by: George Spelvin <lkml@xxxxxxx>
---
drivers/md/bcache/request.c | 15 ++++++++-------
drivers/md/bcache/request.h | 2 +-
drivers/md/bcache/util.h | 26 +++++++++++++++++++-------
3 files changed, 28 insertions(+), 15 deletions(-)
This is quite a drive-by patch; I don't even use bcache or have an SSD to
test it on. But I happened to be looking at the code and was scratching
my head trying to figure out what the heck bch_get_congested() was doing.
This patch is the result of that analysis. Feel free to take whatever
part of it is useful to you.
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 15070412a32e..f1e465c32288 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -329,12 +329,13 @@ void bch_data_insert(struct closure *cl)
bch_data_insert_start(cl);
}
-/* Congested? */
-
-unsigned int bch_get_congested(struct cache_set *c)
+/*
+ * Congested? Return 0 (not congested) or the limit (in sectors)
+ * beyond which we should bypass the cache due to congestion.
+ */
+unsigned int bch_get_congested(const struct cache_set *c)
{
int i;
- long rand;
if (!c->congested_read_threshold_us &&
!c->congested_write_threshold_us)
@@ -353,8 +354,7 @@ unsigned int bch_get_congested(struct cache_set *c)
if (i > 0)
i = fract_exp_two(i, 6);
- rand = get_random_int();
- i -= bitmap_weight(&rand, BITS_PER_LONG);
+ i -= hweight32(get_random_u32());
return i > 0 ? i : 1;
}
@@ -376,7 +376,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
{
struct cache_set *c = dc->disk.c;
unsigned int mode = cache_mode(dc);
- unsigned int sectors, congested = bch_get_congested(c);
+ unsigned int sectors, congested;
struct task_struct *task = current;
struct io *i;
@@ -411,6 +411,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
goto rescale;
}
+ congested = bch_get_congested(c);
if (!congested && !dc->sequential_cutoff)
goto rescale;
diff --git a/drivers/md/bcache/request.h b/drivers/md/bcache/request.h
index 721bf336ed1a..c64dbd7a91aa 100644
--- a/drivers/md/bcache/request.h
+++ b/drivers/md/bcache/request.h
@@ -33,7 +33,7 @@ struct data_insert_op {
BKEY_PADDED(replace_key);
};
-unsigned int bch_get_congested(struct cache_set *c);
+unsigned int bch_get_congested(const struct cache_set *c);
void bch_data_insert(struct closure *cl);
void bch_cached_dev_request_init(struct cached_dev *dc);
diff --git a/drivers/md/bcache/util.h b/drivers/md/bcache/util.h
index 00aab6abcfe4..1fbced94e4cc 100644
--- a/drivers/md/bcache/util.h
+++ b/drivers/md/bcache/util.h
@@ -560,17 +560,29 @@ static inline uint64_t bch_crc64_update(uint64_t crc,
return crc;
}
-/* Does linear interpolation between powers of two */
+/*
+ * A stepwise-linear pseudo-exponential. This returns 1 << (x >>
+ * frac_bits), with the less-significant bits filled in by linear
+ * interpolation.
+ *
+ * This can also be interpreted as a floating-point number format,
+ * where the low frac_bits are the mantissa (with implicit leading
+ * 1 bit), and the more significant bits are the exponent.
+ * The return value is 1.mantissa * 2^exponent.
+ *
+ * The way this is used, fract_bits is 6 and the largest possible
+ * input is CONGESTED_MAX-1 = 1023 (exponent 16, mantissa 0x1.fc),
+ * so the maximum output is 0x1fc00.
+ */
static inline unsigned int fract_exp_two(unsigned int x,
unsigned int fract_bits)
{
- unsigned int fract = x & ~(~0 << fract_bits);
+ unsigned int mantissa = 1 << fract_bits; /* Implicit bit */
- x >>= fract_bits;
- x = 1 << x;
- x += (x * fract) >> fract_bits;
-
- return x;
+ mantissa += x & (mantissa - 1);
+ x >>= fract_bits; /* The exponent */
+ /* Largest intermediate value 0x7f0000 */
+ return mantissa << x >> fract_bits;
}
void bch_bio_map(struct bio *bio, void *base);
--
2.20.1
