* [RFC PATCH v1 06/50] ubi/debug: Optimize computation of odds
@ 2019-03-18 8:22 George Spelvin
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From: George Spelvin @ 2019-03-18 8:22 UTC (permalink / raw)
To: linux-kernel, lkml
Cc: Richard Weinberger, linux-mtd, Adrian Hunter, Artem Bityutskiy
Not only is "prandom_u32() % d" an inefficient way to obtain
a random number, but "prandom_u32_max(d) < n" is an inefficient
way to return true with probability n/d.
Where n and d are compile-time constants, the efficient way to
do this test is "prandom_u32() < ((u64)n << 32)/d.
If n == 1 and d is not a power of 2, then 0xffffffff/d == 0x100000000/d
works just as well and avoids some 64-bit arithmetic.
Signed-off-by: George Spelvin <lkml@sdf.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: linux-mtd@lists.infradead.org
---
drivers/mtd/ubi/debug.h | 6 +++---
fs/ubifs/debug.c | 5 +++--
2 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/drivers/mtd/ubi/debug.h b/drivers/mtd/ubi/debug.h
index 118248a5d7d48..d19671c1a098c 100644
--- a/drivers/mtd/ubi/debug.h
+++ b/drivers/mtd/ubi/debug.h
@@ -73,7 +73,7 @@ static inline int ubi_dbg_is_bgt_disabled(const struct ubi_device *ubi)
static inline int ubi_dbg_is_bitflip(const struct ubi_device *ubi)
{
if (ubi->dbg.emulate_bitflips)
- return !(prandom_u32() % 200);
+ return prandom_u32() < 0xffffffff/200;
return 0;
}
@@ -87,7 +87,7 @@ static inline int ubi_dbg_is_bitflip(const struct ubi_device *ubi)
static inline int ubi_dbg_is_write_failure(const struct ubi_device *ubi)
{
if (ubi->dbg.emulate_io_failures)
- return !(prandom_u32() % 500);
+ return prandom_u32() < 0xffffffff/500;
return 0;
}
@@ -101,7 +101,7 @@ static inline int ubi_dbg_is_write_failure(const struct ubi_device *ubi)
static inline int ubi_dbg_is_erase_failure(const struct ubi_device *ubi)
{
if (ubi->dbg.emulate_io_failures)
- return !(prandom_u32() % 400);
+ return prandom_u32() < 0xffffffff/400;
return 0;
}
diff --git a/fs/ubifs/debug.c b/fs/ubifs/debug.c
index 0f5a480fe264f..3d8d8eaea6c66 100644
--- a/fs/ubifs/debug.c
+++ b/fs/ubifs/debug.c
@@ -2444,9 +2444,10 @@ int dbg_check_nondata_nodes_order(struct ubifs_info *c, struct list_head *head)
return 0;
}
-static inline int chance(unsigned int n, unsigned int out_of)
+static bool chance(unsigned int n, unsigned int out_of)
{
- return !!((prandom_u32() % out_of) + 1 <= n);
+ /* RHS is a constant expression */
+ return prandom_u32() < ((u64)n << 32) / out_of;
}
--
2.26.0
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