diff options
author | George Spelvin <lkml@sdf.org> | 2019-04-25 00:48:30 +0800 |
---|---|---|
committer | Jens Axboe <axboe@kernel.dk> | 2019-04-24 10:56:27 -0600 |
commit | 3a3947271cd6ce05c5b30c1250fa99de57410500 (patch) | |
tree | 91649ddb68a097e002c3d7d3209de1d2eeea53e6 /drivers/md/bcache/request.h | |
parent | 792732d9852c0e4505aceff4631ea2168fd02480 (diff) |
bcache: Clean up bch_get_congested()
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>
Diffstat (limited to 'drivers/md/bcache/request.h')
-rw-r--r-- | drivers/md/bcache/request.h | 2 |
1 files changed, 1 insertions, 1 deletions
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); |