From: George Spelvin lkml@sdf.org
mainline inclusion from mainline-5.2-rc1 commit 3a3947271cd6ce05c5b30c1250fa99de57410500 category: backport
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@sdf.org Signed-off-by: Coly Li colyli@suse.de Signed-off-by: Jens Axboe axboe@kernel.dk --- 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(-)
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c index f11123079fe0..41adcd1546f1 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;
@@ -412,6 +412,7 @@ 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 b1f5b7aea872..c029f7443190 100644 --- a/drivers/md/bcache/util.h +++ b/drivers/md/bcache/util.h @@ -558,17 +558,29 @@ 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); - - x >>= fract_bits; - x = 1 << x; - x += (x * fract) >> fract_bits; + unsigned int mantissa = 1 << fract_bits; /* Implicit bit */
- 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);