lkml.org 
[lkml]   [2002]   [Jun]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    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;
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/
    \
     
     \ /
      Last update: 2005-03-22 13:26    [W:0.052 / U:1.748 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site