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Subject[PATCH] poll/select fast path

This patch streamlines poll and select by adding fast paths for a
small number of descriptors passed. The majority of polls/selects
seem to be of this nature. The main saving comes from not allocating
two pages for wait queue and table, but from using stack allocation
(upto 256bytes) when only a few descriptors are needed. This makes
it as fast again as 2.0 and even a bit faster because the wait queue
page allocation is avoided too (except when the drivers overflow it)

select also skips a lot faster over big holes and avoids the separate
pass of determining the max. number of descriptors in the bitmap.

a typical linux system saves a considerable amount of unswappable memory
with this patch, because it usually has 10+ daemons hanging around in poll or
select with each two pages allocated for data and wait queue.

Some other cleanups.

Patch for 2.5.22.

-Andi


--- linux/include/linux/poll.h Mon Jun 17 06:25:51 2002
+++ linux-2.5.22-work/include/linux/poll.h Mon Jun 17 08:32:05 2002
@@ -10,13 +10,32 @@
#include <linux/mm.h>
#include <asm/uaccess.h>

-struct poll_table_page;
+#define POLL_INLINE_BYTES 256
+#define FAST_SELECT_MAX 128
+#define FAST_POLL_MAX 128
+#define POLL_INLINE_ENTRIES (1+(POLL_INLINE_BYTES / sizeof(struct poll_table_entry)))
+
+struct poll_table_entry {
+ struct file * filp;
+ wait_queue_t wait;
+ wait_queue_head_t * wait_address;
+};
+
+struct poll_table_page {
+ struct poll_table_page * next;
+ struct poll_table_entry * entry;
+ struct poll_table_entry entries[0];
+};

typedef struct poll_table_struct {
int error;
struct poll_table_page * table;
+ struct poll_table_page inline_page;
+ struct poll_table_entry inline_table[POLL_INLINE_ENTRIES];
} poll_table;

+#define POLL_INLINE_TABLE_LEN (sizeof(poll_table) - offsetof(poll_table, inline_page))
+
extern void __pollwait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p);

static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
@@ -30,6 +49,7 @@
pt->error = 0;
pt->table = NULL;
}
+
extern void poll_freewait(poll_table* pt);


@@ -49,38 +69,11 @@
#define FDS_LONGS(nr) (((nr)+FDS_BITPERLONG-1)/FDS_BITPERLONG)
#define FDS_BYTES(nr) (FDS_LONGS(nr)*sizeof(long))

-/*
- * We do a VERIFY_WRITE here even though we are only reading this time:
- * we'll write to it eventually..
- *
- * Use "unsigned long" accesses to let user-mode fd_set's be long-aligned.
- */
-static inline
-int get_fd_set(unsigned long nr, void *ufdset, unsigned long *fdset)
-{
- nr = FDS_BYTES(nr);
- if (ufdset) {
- int error;
- error = verify_area(VERIFY_WRITE, ufdset, nr);
- if (!error && __copy_from_user(fdset, ufdset, nr))
- error = -EFAULT;
- return error;
- }
- memset(fdset, 0, nr);
- return 0;
-}
-
static inline
void set_fd_set(unsigned long nr, void *ufdset, unsigned long *fdset)
{
if (ufdset)
__copy_to_user(ufdset, fdset, FDS_BYTES(nr));
-}
-
-static inline
-void zero_fd_set(unsigned long nr, unsigned long *fdset)
-{
- memset(fdset, 0, FDS_BYTES(nr));
}

extern int do_select(int n, fd_set_bits *fds, long *timeout);
--- linux/fs/select.c Mon Jun 17 06:25:40 2002
+++ linux-2.5.22-work/fs/select.c Tue Jun 18 09:58:44 2002
@@ -12,6 +12,9 @@
* 24 January 2000
* Changed sys_poll()/do_poll() to use PAGE_SIZE chunk-based allocation
* of fds to overcome nfds < 16390 descriptors limit (Tigran Aivazian).
+ *
+ * Dec 2001
+ * Stack allocation and fast path (Andi Kleen)
*/

#include <linux/slab.h>
@@ -26,21 +29,6 @@
#define ROUND_UP(x,y) (((x)+(y)-1)/(y))
#define DEFAULT_POLLMASK (POLLIN | POLLOUT | POLLRDNORM | POLLWRNORM)

-struct poll_table_entry {
- struct file * filp;
- wait_queue_t wait;
- wait_queue_head_t * wait_address;
-};
-
-struct poll_table_page {
- struct poll_table_page * next;
- struct poll_table_entry * entry;
- struct poll_table_entry entries[0];
-};
-
-#define POLL_TABLE_FULL(table) \
- ((unsigned long)((table)->entry+1) > PAGE_SIZE + (unsigned long)(table))
-
/*
* Ok, Peter made a complicated, but straightforward multiple_wait() function.
* I have rewritten this, taking some shortcuts: This code may not be easy to
@@ -62,30 +50,39 @@
struct poll_table_page *old;

entry = p->entry;
- do {
+ while (entry > p->entries) {
entry--;
remove_wait_queue(entry->wait_address,&entry->wait);
fput(entry->filp);
- } while (entry > p->entries);
+ }
old = p;
p = p->next;
- free_page((unsigned long) old);
+ if (old != &pt->inline_page)
+ free_page((unsigned long) old);
}
}

void __pollwait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
struct poll_table_page *table = p->table;
+ struct poll_table_page *new_table = NULL;
+ int sz;

- if (!table || POLL_TABLE_FULL(table)) {
- struct poll_table_page *new_table;
-
- new_table = (struct poll_table_page *) __get_free_page(GFP_KERNEL);
- if (!new_table) {
- p->error = -ENOMEM;
- __set_current_state(TASK_RUNNING);
- return;
+ if (!table) {
+ new_table = &p->inline_page;
+ } else {
+ sz = (table == &p->inline_page) ? POLL_INLINE_TABLE_LEN : PAGE_SIZE;
+ if ((char*)table->entry >= (char*)table + sz) {
+ new_table = (struct poll_table_page *)__get_free_page(GFP_KERNEL);
+ if (!new_table) {
+ p->error = -ENOMEM;
+ __set_current_state(TASK_RUNNING);
+ return;
+ }
}
+ }
+
+ if (new_table) {
new_table->entry = new_table->entries;
new_table->next = table;
p->table = new_table;
@@ -113,48 +110,6 @@

#define BITS(fds, n) (*__IN(fds, n)|*__OUT(fds, n)|*__EX(fds, n))

-static int max_select_fd(unsigned long n, fd_set_bits *fds)
-{
- unsigned long *open_fds;
- unsigned long set;
- int max;
-
- /* handle last in-complete long-word first */
- set = ~(~0UL << (n & (__NFDBITS-1)));
- n /= __NFDBITS;
- open_fds = current->files->open_fds->fds_bits+n;
- max = 0;
- if (set) {
- set &= BITS(fds, n);
- if (set) {
- if (!(set & ~*open_fds))
- goto get_max;
- return -EBADF;
- }
- }
- while (n) {
- open_fds--;
- n--;
- set = BITS(fds, n);
- if (!set)
- continue;
- if (set & ~*open_fds)
- return -EBADF;
- if (max)
- continue;
-get_max:
- do {
- max++;
- set >>= 1;
- } while (set);
- max += n * __NFDBITS;
- }
-
- return max;
-}
-
-#define BIT(i) (1UL << ((i)&(__NFDBITS-1)))
-#define MEM(i,m) ((m)+(unsigned)(i)/__NFDBITS)
#define ISSET(i,m) (((i)&*(m)) != 0)
#define SET(i,m) (*(m) |= (i))

@@ -165,56 +120,71 @@
int do_select(int n, fd_set_bits *fds, long *timeout)
{
poll_table table, *wait;
- int retval, i, off;
+ int retval, off, max, maxoff;
long __timeout = *timeout;

- read_lock(&current->files->file_lock);
- retval = max_select_fd(n, fds);
- read_unlock(&current->files->file_lock);
-
- if (retval < 0)
- return retval;
- n = retval;
-
poll_initwait(&table);
wait = &table;
if (!__timeout)
wait = NULL;
+
retval = 0;
+ maxoff = n/BITS_PER_LONG;
+ max = 0;
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
- for (i = 0 ; i < n; i++) {
- unsigned long bit = BIT(i);
- unsigned long mask;
- struct file *file;
+ for (off = 0; off <= maxoff; off++) {
+ unsigned long val = BITS(fds, off);

- off = i / __NFDBITS;
- if (!(bit & BITS(fds, off)))
+ if (!val)
continue;
- file = fget(i);
- mask = POLLNVAL;
- if (file) {
- mask = DEFAULT_POLLMASK;
- if (file->f_op && file->f_op->poll)
- mask = file->f_op->poll(file, wait);
- fput(file);
- }
- if ((mask & POLLIN_SET) && ISSET(bit, __IN(fds,off))) {
- SET(bit, __RES_IN(fds,off));
- retval++;
- wait = NULL;
- }
- if ((mask & POLLOUT_SET) && ISSET(bit, __OUT(fds,off))) {
- SET(bit, __RES_OUT(fds,off));
- retval++;
- wait = NULL;
- }
- if ((mask & POLLEX_SET) && ISSET(bit, __EX(fds,off))) {
- SET(bit, __RES_EX(fds,off));
- retval++;
- wait = NULL;
+ while (val) {
+ int k = ffz(~val);
+ unsigned long mask, bit;
+ struct file *file;
+
+ if (k > n%BITS_PER_LONG)
+ break;
+
+ bit = (1UL << k);
+ val &= ~bit;
+
+ file = fget((off * BITS_PER_LONG) + k);
+ mask = POLLNVAL;
+ if (file) {
+ mask = DEFAULT_POLLMASK;
+ if (file->f_op && file->f_op->poll)
+ mask = file->f_op->poll(file, wait);
+ fput(file);
+ } else {
+ /* This error will shadow all other results.
+ * This matches previous linux behaviour */
+ retval = -EBADF;
+ goto out;
+ }
+ if ((mask & POLLIN_SET) && ISSET(bit, __IN(fds,off))) {
+ SET(bit, __RES_IN(fds,off));
+ retval++;
+ wait = NULL;
+ }
+ if ((mask& POLLOUT_SET) && ISSET(bit,__OUT(fds,off))) {
+ SET(bit, __RES_OUT(fds,off));
+ retval++;
+ wait = NULL;
+ }
+ if ((mask & POLLEX_SET) && ISSET(bit, __EX(fds,off))) {
+ SET(bit, __RES_EX(fds,off));
+ retval++;
+ wait = NULL;
+ }
+
+ if (!(val &= ~bit))
+ break;
}
}
+
+
+ maxoff = max;
wait = NULL;
if (retval || !__timeout || signal_pending(current))
break;
@@ -224,25 +194,43 @@
}
__timeout = schedule_timeout(__timeout);
}
+
+out:
current->state = TASK_RUNNING;

poll_freewait(&table);

/*
- * Up-to-date the caller timeout.
+ * Update the caller timeout.
*/
*timeout = __timeout;
return retval;
}

-static void *select_bits_alloc(int size)
-{
- return kmalloc(6 * size, GFP_KERNEL);
-}
+/*
+ * We do a VERIFY_WRITE here even though we are only reading this time:
+ * we'll write to it eventually..
+ */

-static void select_bits_free(void *bits, int size)
+static int get_fd_set(unsigned long nr, void *ufdset, unsigned long *fdset)
{
- kfree(bits);
+ unsigned long rounded = FDS_BYTES(nr), mask;
+ if (ufdset) {
+ int error = verify_area(VERIFY_WRITE, ufdset, rounded);
+ if (!error && __copy_from_user(fdset, ufdset, rounded))
+ error = -EFAULT;
+ if (nr % __NFDBITS == 0)
+ mask = 0;
+ else {
+ /* This includes one bit too much according to SU;
+ but without this some programs hang. */
+ mask = ~(~0UL << (nr%__NFDBITS));
+ }
+ fdset[nr/__NFDBITS] &= mask;
+ return error;
+ }
+ memset(fdset, 0, rounded);
+ return 0;
}

/*
@@ -263,6 +251,7 @@
char *bits;
long timeout;
int ret, size, max_fdset;
+ char stack_bits[FDS_BYTES(FAST_SELECT_MAX) * 6];

timeout = MAX_SCHEDULE_TIMEOUT;
if (tvp) {
@@ -297,11 +286,16 @@
* since we used fdset we need to allocate memory in units of
* long-words.
*/
- ret = -ENOMEM;
size = FDS_BYTES(n);
- bits = select_bits_alloc(size);
- if (!bits)
- goto out_nofds;
+ if (n < FAST_SELECT_MAX) {
+ bits = stack_bits;
+ } else {
+ ret = -ENOMEM;
+ bits = kmalloc(6*size, GFP_KERNEL);
+ if (!bits)
+ goto out_nofds;
+ }
+
fds.in = (unsigned long *) bits;
fds.out = (unsigned long *) (bits + size);
fds.ex = (unsigned long *) (bits + 2*size);
@@ -313,9 +307,7 @@
(ret = get_fd_set(n, outp, fds.out)) ||
(ret = get_fd_set(n, exp, fds.ex)))
goto out;
- zero_fd_set(n, fds.res_in);
- zero_fd_set(n, fds.res_out);
- zero_fd_set(n, fds.res_ex);
+ memset(fds.res_in, 0, 3*size);

ret = do_select(n, &fds, &timeout);

@@ -326,8 +318,8 @@
usec = timeout % HZ;
usec *= (1000000/HZ);
}
- put_user(sec, &tvp->tv_sec);
- put_user(usec, &tvp->tv_usec);
+ __put_user(sec, &tvp->tv_sec);
+ __put_user(usec, &tvp->tv_usec);
}

if (ret < 0)
@@ -344,8 +336,10 @@
set_fd_set(n, exp, fds.res_ex);

out:
- select_bits_free(bits, size);
+ if (n >= FAST_SELECT_MAX)
+ kfree(bits);
out_nofds:
+
return ret;
}

@@ -410,12 +404,42 @@
return count;
}

+static int fast_poll(poll_table *table, poll_table *wait, struct pollfd *ufds,
+ unsigned int nfds, long timeout)
+{
+ poll_table *pt = wait;
+ struct pollfd fds[FAST_POLL_MAX];
+ int count, i;
+
+ if (copy_from_user(fds, ufds, nfds * sizeof(struct pollfd)))
+ return -EFAULT;
+ for (;;) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ count = 0;
+ do_pollfd(nfds, fds, &pt, &count);
+ pt = NULL;
+ if (count || !timeout || signal_pending(current))
+ break;
+ count = wait->error;
+ if (count)
+ break;
+ timeout = schedule_timeout(timeout);
+ }
+ current->state = TASK_RUNNING;
+ for (i = 0; i < nfds; i++)
+ __put_user(fds[i].revents, &ufds[i].revents);
+ poll_freewait(table);
+ if (!count && signal_pending(current))
+ return -EINTR;
+ return count;
+}
+
asmlinkage long sys_poll(struct pollfd * ufds, unsigned int nfds, long timeout)
{
- int i, j, fdcount, err;
+ int i, j, err, fdcount;
struct pollfd **fds;
poll_table table, *wait;
- int nchunks, nleft;
+ int nchunks, nleft;

/* Do a sanity check on nfds ... */
if (nfds > NR_OPEN)
@@ -429,43 +453,45 @@
timeout = MAX_SCHEDULE_TIMEOUT;
}

+
poll_initwait(&table);
wait = &table;
if (!timeout)
wait = NULL;

- err = -ENOMEM;
- fds = NULL;
- if (nfds != 0) {
- fds = (struct pollfd **)kmalloc(
- (1 + (nfds - 1) / POLLFD_PER_PAGE) * sizeof(struct pollfd *),
- GFP_KERNEL);
- if (fds == NULL)
- goto out;
- }
+ if (nfds < FAST_POLL_MAX)
+ return fast_poll(&table, wait, ufds, nfds, timeout);

+ err = -ENOMEM;
+ fds = (struct pollfd **)kmalloc(
+ (1 + (nfds - 1) / POLLFD_PER_PAGE) * sizeof(struct pollfd *),
+ GFP_KERNEL);
+ if (fds == NULL)
+ goto out;
+
nchunks = 0;
nleft = nfds;
- while (nleft > POLLFD_PER_PAGE) { /* allocate complete PAGE_SIZE chunks */
+ while (nleft > POLLFD_PER_PAGE) {
fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL);
if (fds[nchunks] == NULL)
goto out_fds;
nchunks++;
nleft -= POLLFD_PER_PAGE;
}
- if (nleft) { /* allocate last PAGE_SIZE chunk, only nleft elements used */
+ if (nleft) {
fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL);
if (fds[nchunks] == NULL)
goto out_fds;
- }
-
+ }
+
err = -EFAULT;
for (i=0; i < nchunks; i++)
if (copy_from_user(fds[i], ufds + i*POLLFD_PER_PAGE, PAGE_SIZE))
goto out_fds1;
+
if (nleft) {
if (copy_from_user(fds[nchunks], ufds + nchunks*POLLFD_PER_PAGE,
- nleft * sizeof(struct pollfd)))
+ nleft * sizeof(struct pollfd)))
goto out_fds1;
}

@@ -489,8 +515,7 @@
out_fds:
for (i=0; i < nchunks; i++)
free_page((unsigned long)(fds[i]));
- if (nfds != 0)
- kfree(fds);
+ kfree(fds);
out:
poll_freewait(&table);
return err;
-
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