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SubjectRe: [PATCH] Linux-2.5 fix/improve get_pid()
Paul Larson wrote:
>
> This patch provides an improved version of get_pid() while also taking
> care of the bug that causes the machine to hang when you hit PID_MAX.

Has this been evaluated against Bill Irwin's constant-time
allocator? Bill says it has slightly worse normal-case and
vastly improved worst-case performance over the stock allocator.
Not sure how it stacks up against this one. Plus it's much nicer
looking code.

He's shy, so.....



#include <linux/compiler.h>
#include <linux/bitops.h>
#include <linux/spinlock.h>
#include <linux/init.h>

/*
* pid allocator
* (C) 2002 William Irwin, IBM
*
* The strategy is to maintain a tower of bitmaps where a bitmap above
* another in each bit accounts whether any pid's are available in the
* space tracked by BITS_PER_LONG bits of the level below. The bitmaps
* must be marked on allocation and also release, hence some
* infrastructure for detecting when the last user of a pid releases it
* must be in place.
*
* This general strategy is simple in concept and enforces highly
* deterministic bounds on the search time for the next pid.
*/

#define PID_MAX 0x8000
#define RESERVED_PIDS 300

#define MAP0_SIZE (PID_MAX >> BITS_PER_LONG_SHIFT)
#define MAP1_SIZE (MAP0_SIZE >> BITS_PER_LONG_SHIFT)
#define MAP2_SIZE (MAP1_SIZE >> BITS_PER_LONG_SHIFT)

#define MAP0_SHIFT BITS_PER_LONG_SHIFT
#define MAP1_SHIFT (2*BITS_PER_LONG_SHIFT)
#define MAP2_SHIFT (3*BITS_PER_LONG_SHIFT)

#define PID_MAP_MASK (BITS_PER_LONG - 1)

#define PID_MAP_DEPTH (ARRAY_SIZE(pid_map) - 1)

static unsigned long pid_map0[MAP0_SIZE];
static unsigned long pid_map1[MAP1_SIZE];
static unsigned long pid_map2[MAP2_SIZE];

static unsigned long * pid_map[] = { pid_map0, pid_map1, pid_map2, NULL, };

unsigned long last_pid = 0;
unsigned long npids = 0;

static const int map_shifts[] =
{ 0,
BITS_PER_LONG_SHIFT,
BITS_PER_LONG_SHIFT*2,
BITS_PER_LONG_SHIFT*3,
BITS_PER_LONG_SHIFT*4,
};

static inline int pid_map_shift(int depth)
{
return map_shifts[depth+1];
}

static spinlock_t pid_lock = SPIN_LOCK_UNLOCKED;

void free_pid(unsigned long pid)
{
unsigned long **map = pid_map;

spin_lock(&pid_lock);
while (*map) {
int bit = pid & PID_MAP_MASK;
pid >>= BITS_PER_LONG_SHIFT;
__clear_bit(bit, &(*map)[pid]);
++map;
}
--npids;
spin_unlock(&pid_lock);
}

static inline int whole_block_used(int level, unsigned long pid)
{
return pid_map[level][pid >> pid_map_shift(level)] == ~0UL;
}

static inline void mark_pid(unsigned long pid)
{
int level;
for (level = 0; level < PID_MAP_DEPTH; ++level) {
int shift, bit;
unsigned long entry;

shift = pid_map_shift(level);
entry = pid >> shift;
bit = (pid >> (shift - BITS_PER_LONG_SHIFT)) & PID_MAP_MASK;
if (level == 0 || whole_block_used(level - 1, pid))
__set_bit(bit, &pid_map[level][entry]);
else
break;
}
++npids;
}

static inline int pid_map_limit(int depth)
{
return PID_MAX >> pid_map_shift(depth);
}

#ifdef PID_ALLOC_EXAMPLE
/*
* the pid allocation traverses the bitmaps by recursively ffz'ing
* through down the tower of maps. Some additional logic is required
* to enforce lower limits, but the following example of how one
* would perform this search without the lower limit may well prove
* enlightening for those interested in the mechanics of the algorithm.
*/
static long alloc_pid_from_zero(void)
{
unsigned long pid = 0;
int level;

for (level = PID_MAP_DEPTH - 1; level >= 0; --level) {
unsigned long entry = pid_map[level][pid];

if (unlikely(entry == ~0UL))
return ~0UL;

pid = (pid << BITS_PER_LONG_SHIFT) + ffz(pid_map[level][pid]);
}
return pid;
}
#endif /* PID_ALLOC_EXAMPLE */


static const unsigned long pid_max_levels[] =
{ PID_MAX >> BITS_PER_LONG_SHIFT,
PID_MAX >> (BITS_PER_LONG_SHIFT*2),
PID_MAX >> (BITS_PER_LONG_SHIFT*3),
PID_MAX >> (BITS_PER_LONG_SHIFT*4),
};

static inline unsigned long pid_map_digit(int level, unsigned long limit)
{
return (limit >> pid_map_shift(level-1)) & PID_MAP_MASK;
}

/*
* Scratch space for storing the digits of the limit, all accesses
* protected by the pid_lock.
*/
static unsigned long limit_digits[4];

/*
* This is not a high-performance implementation. alloc_pid_after()
* can be made highly compact with some effort, but this is instead
* meant to be clear. As the cost of fork() is dominated by much
* more expensive operations and the cost of this is constant-bounded
* by a very low constant, the gains from manual optimization here
* are marginal.
*/
static long alloc_pid_after(unsigned long limit)
{
unsigned long pid = 0;
int level;

/*
* The limit passed to us is a strict lower limit. It is more
* convenient to work with <= constraints.
*/
++limit;
if (unlikely(limit == PID_MAX))
return ~0UL;

for (level = 0; level < PID_MAP_DEPTH; ++level) {
limit_digits[level] = limit & PID_MAP_MASK;
limit >>= BITS_PER_LONG_SHIFT;
}

/*
* Now the lowest available pid number above limit is
* reconstructed by ffz'ing down the bitmap and checking
* each digit against the digits of the limit for
* dictionary ordering. If the check should fail, it's
* fixed up by using the maximum of the two digits. The
* dictionary ordering on digits also means that a
* greater significant digit found in the bitmap
* invalidates all further comparisons, which requires
* fallback to the pure recursive ffz algorithm outlined
* above in order to be handled.
*/
for (level = PID_MAP_DEPTH - 1; level >= 0; --level) {
unsigned long bit, digit;

if (unlikely(pid >= pid_max_levels[level]))
return ~0UL;

bit = ffz(pid_map[level][pid]);
digit = limit_digits[level];

if (unlikely(bit < digit))
bit = digit;

pid = (pid << BITS_PER_LONG_SHIFT) + bit;

/*
* This is not an optimization; if this check
* should succeed the digit comparisons above
* are no longer valid and (pessimistically)
* incorrect first available pid's are found.
*
*/
if (likely(bit > digit)) {
--level;
goto finish_just_ffz;
}
}
out:
if (pid < PID_MAX)
return pid;
else
return ~0UL;

finish_just_ffz:
/*
* Now revert to the pure recursive ffz algorithm with
* the slight variation of not beginning at a fixed level,
* because it is no longer valid to perform comparisons
* of the digit obtained by ffz'ing the bitmap against the
* digits of the limit.
*/
while (level >= 0) {
unsigned long bit;

if (unlikely(pid >= pid_max_levels[level]))
return ~0UL;

bit = ffz(pid_map[level][pid]);
pid = (pid << BITS_PER_LONG_SHIFT) + bit;
--level;
}

goto out;
}

int alloc_pid(void)
{
unsigned long pid;

spin_lock(&pid_lock);
BUG_ON(last_pid >= PID_MAX);
pid = alloc_pid_after(last_pid);
if (unlikely(pid == ~0UL)) {
pid = alloc_pid_after(RESERVED_PIDS);
if (unlikely(pid == ~0UL))
goto out;
BUG_ON(pid < RESERVED_PIDS);
} else
BUG_ON(pid <= last_pid);
last_pid = pid;
mark_pid(pid);
out:
spin_unlock(&pid_lock);
return (int)pid;
}

void __init pid_init(void)
{
mark_pid(0);
}
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