pipe: use exclusive waits when reading or writing

This makes the pipe code use separate wait-queues and exclusive waiting
for readers and writers, avoiding a nasty thundering herd problem when
there are lots of readers waiting for data on a pipe (or, less commonly,
lots of writers waiting for a pipe to have space).

While this isn't a common occurrence in the traditional "use a pipe as a
data transport" case, where you typically only have a single reader and
a single writer process, there is one common special case: using a pipe
as a source of "locking tokens" rather than for data communication.

In particular, the GNU make jobserver code ends up using a pipe as a way
to limit parallelism, where each job consumes a token by reading a byte
from the jobserver pipe, and releases the token by writing a byte back
to the pipe.

This pattern is fairly traditional on Unix, and works very well, but
will waste a lot of time waking up a lot of processes when only a single
reader needs to be woken up when a writer releases a new token.

A simplified test-case of just this pipe interaction is to create 64
processes, and then pass a single token around between them (this
test-case also intentionally passes another token that gets ignored to
test the "wake up next" logic too, in case anybody wonders about it):

    #include <unistd.h>

    int main(int argc, char **argv)
    {
        int fd[2], counters[2];

        pipe(fd);
        counters[0] = 0;
        counters[1] = -1;
        write(fd[1], counters, sizeof(counters));

        /* 64 processes */
        fork(); fork(); fork(); fork(); fork(); fork();

        do {
                int i;
                read(fd[0], &i, sizeof(i));
                if (i < 0)
                        continue;
                counters[0] = i+1;
                write(fd[1], counters, (1+(i & 1)) *sizeof(int));
        } while (counters[0] < 1000000);
        return 0;
    }

and in a perfect world, passing that token around should only cause one
context switch per transfer, when the writer of a token causes a
directed wakeup of just a single reader.

But with the "writer wakes all readers" model we traditionally had, on
my test box the above case causes more than an order of magnitude more
scheduling: instead of the expected ~1M context switches, "perf stat"
shows

        231,852.37 msec task-clock                #   15.857 CPUs utilized
        11,250,961      context-switches          #    0.049 M/sec
           616,304      cpu-migrations            #    0.003 M/sec
             1,648      page-faults               #    0.007 K/sec
 1,097,903,998,514      cycles                    #    4.735 GHz
   120,781,778,352      instructions              #    0.11  insn per cycle
    27,997,056,043      branches                  #  120.754 M/sec
       283,581,233      branch-misses             #    1.01% of all branches

      14.621273891 seconds time elapsed

       0.018243000 seconds user
       3.611468000 seconds sys

before this commit.

After this commit, I get

          5,229.55 msec task-clock                #    3.072 CPUs utilized
         1,212,233      context-switches          #    0.232 M/sec
           103,951      cpu-migrations            #    0.020 M/sec
             1,328      page-faults               #    0.254 K/sec
    21,307,456,166      cycles                    #    4.074 GHz
    12,947,819,999      instructions              #    0.61  insn per cycle
     2,881,985,678      branches                  #  551.096 M/sec
        64,267,015      branch-misses             #    2.23% of all branches

       1.702148350 seconds time elapsed

       0.004868000 seconds user
       0.110786000 seconds sys

instead. Much better.

[ Note! This kernel improvement seems to be very good at triggering a
  race condition in the make jobserver (in GNU make 4.2.1) for me. It's
  a long known bug that was fixed back in June 2017 by GNU make commit
  b552b0525198 ("[SV 51159] Use a non-blocking read with pselect to
  avoid hangs.").

  But there wasn't a new release of GNU make until 4.3 on Jan 19 2020,
  so a number of distributions may still have the buggy version. Some
  have backported the fix to their 4.2.1 release, though, and even
  without the fix it's quite timing-dependent whether the bug actually
  is hit. ]

Josh Triplett says:
 "I've been hammering on your pipe fix patch (switching to exclusive
  wait queues) for a month or so, on several different systems, and I've
  run into no issues with it. The patch *substantially* improves
  parallel build times on large (~100 CPU) systems, both with parallel
  make and with other things that use make's pipe-based jobserver.

  All current distributions (including stable and long-term stable
  distributions) have versions of GNU make that no longer have the
  jobserver bug"

Tested-by: Josh Triplett <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

fs/coredump.c

@@ -517,15 +517,15 @@ static void wait_for_dump_helpers(struct file *file)
 	pipe_lock(pipe);
 	pipe->readers++;
 	pipe->writers--;
-	wake_up_interruptible_sync(&pipe->wait);
+	wake_up_interruptible_sync(&pipe->rd_wait);
 	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	pipe_unlock(pipe);
 
 	/*
 	 * We actually want wait_event_freezable() but then we need
 	 * to clear TIF_SIGPENDING and improve dump_interrupted().
 	 */
-	wait_event_interruptible(pipe->wait, pipe->readers == 1);
+	wait_event_interruptible(pipe->rd_wait, pipe->readers == 1);
 
 	pipe_lock(pipe);
 	pipe->readers--;

fs/pipe.c

@@ -108,16 +108,19 @@ void pipe_double_lock(struct pipe_inode_info *pipe1,
 /* Drop the inode semaphore and wait for a pipe event, atomically */
 void pipe_wait(struct pipe_inode_info *pipe)
 {
-	DEFINE_WAIT(wait);
+	DEFINE_WAIT(rdwait);
+	DEFINE_WAIT(wrwait);
 
 	/*
 	 * Pipes are system-local resources, so sleeping on them
 	 * is considered a noninteractive wait:
 	 */
-	prepare_to_wait(&pipe->wait, &wait, TASK_INTERRUPTIBLE);
+	prepare_to_wait(&pipe->rd_wait, &rdwait, TASK_INTERRUPTIBLE);
+	prepare_to_wait(&pipe->wr_wait, &wrwait, TASK_INTERRUPTIBLE);
 	pipe_unlock(pipe);
 	schedule();
-	finish_wait(&pipe->wait, &wait);
+	finish_wait(&pipe->rd_wait, &rdwait);
+	finish_wait(&pipe->wr_wait, &wrwait);
 	pipe_lock(pipe);
 }
 
@@ -286,7 +289,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	size_t total_len = iov_iter_count(to);
 	struct file *filp = iocb->ki_filp;
 	struct pipe_inode_info *pipe = filp->private_data;
-	bool was_full;
+	bool was_full, wake_next_reader = false;
 	ssize_t ret;
 
 	/* Null read succeeds. */
@@ -344,10 +347,10 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 
 			if (!buf->len) {
 				pipe_buf_release(pipe, buf);
-				spin_lock_irq(&pipe->wait.lock);
+				spin_lock_irq(&pipe->rd_wait.lock);
 				tail++;
 				pipe->tail = tail;
-				spin_unlock_irq(&pipe->wait.lock);
+				spin_unlock_irq(&pipe->rd_wait.lock);
 			}
 			total_len -= chars;
 			if (!total_len)
@@ -384,7 +387,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		 * no data.
 		 */
 		if (unlikely(was_full)) {
-			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
+			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 		}
 
@@ -394,18 +397,23 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		 * since we've done any required wakeups and there's no need
 		 * to mark anything accessed. And we've dropped the lock.
 		 */
-		if (wait_event_interruptible(pipe->wait, pipe_readable(pipe)) < 0)
+		if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
 			return -ERESTARTSYS;
 
 		__pipe_lock(pipe);
 		was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
+		wake_next_reader = true;
 	}
+	if (pipe_empty(pipe->head, pipe->tail))
+		wake_next_reader = false;
 	__pipe_unlock(pipe);
 
 	if (was_full) {
-		wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
+		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 	}
+	if (wake_next_reader)
+		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 	if (ret > 0)
 		file_accessed(filp);
 	return ret;
@@ -437,6 +445,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	size_t total_len = iov_iter_count(from);
 	ssize_t chars;
 	bool was_empty = false;
+	bool wake_next_writer = false;
 
 	/* Null write succeeds. */
 	if (unlikely(total_len == 0))
@@ -515,16 +524,16 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			 * it, either the reader will consume it or it'll still
 			 * be there for the next write.
 			 */
-			spin_lock_irq(&pipe->wait.lock);
+			spin_lock_irq(&pipe->rd_wait.lock);
 
 			head = pipe->head;
 			if (pipe_full(head, pipe->tail, pipe->max_usage)) {
-				spin_unlock_irq(&pipe->wait.lock);
+				spin_unlock_irq(&pipe->rd_wait.lock);
 				continue;
 			}
 
 			pipe->head = head + 1;
-			spin_unlock_irq(&pipe->wait.lock);
+			spin_unlock_irq(&pipe->rd_wait.lock);
 
 			/* Insert it into the buffer array */
 			buf = &pipe->bufs[head & mask];
@@ -576,14 +585,17 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		 */
 		__pipe_unlock(pipe);
 		if (was_empty) {
-			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM);
+			wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 			kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		}
-		wait_event_interruptible(pipe->wait, pipe_writable(pipe));
+		wait_event_interruptible_exclusive(pipe->wr_wait, pipe_writable(pipe));
 		__pipe_lock(pipe);
 		was_empty = pipe_empty(pipe->head, pipe->tail);
+		wake_next_writer = true;
 	}
 out:
+	if (pipe_full(pipe->head, pipe->tail, pipe->max_usage))
+		wake_next_writer = false;
 	__pipe_unlock(pipe);
 
 	/*
@@ -596,9 +608,11 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	 * wake up pending jobs
 	 */
 	if (was_empty) {
-		wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM);
+		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	}
+	if (wake_next_writer)
+		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 	if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) {
 		int err = file_update_time(filp);
 		if (err)
@@ -642,12 +656,15 @@ pipe_poll(struct file *filp, poll_table *wait)
 	unsigned int head, tail;
 
 	/*
-	 * Reading only -- no need for acquiring the semaphore.
+	 * Reading pipe state only -- no need for acquiring the semaphore.
 	 *
 	 * But because this is racy, the code has to add the
 	 * entry to the poll table _first_ ..
 	 */
-	poll_wait(filp, &pipe->wait, wait);
+	if (filp->f_mode & FMODE_READ)
+		poll_wait(filp, &pipe->rd_wait, wait);
+	if (filp->f_mode & FMODE_WRITE)
+		poll_wait(filp, &pipe->wr_wait, wait);
 
 	/*
 	 * .. and only then can you do the racy tests. That way,
@@ -706,7 +723,8 @@ pipe_release(struct inode *inode, struct file *file)
 		pipe->writers--;
 
 	if (pipe->readers || pipe->writers) {
-		wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLOUT | EPOLLRDNORM | EPOLLWRNORM | EPOLLERR | EPOLLHUP);
+		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM | EPOLLERR | EPOLLHUP);
+		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM | EPOLLERR | EPOLLHUP);
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 	}
@@ -789,7 +807,8 @@ struct pipe_inode_info *alloc_pipe_info(void)
 			     GFP_KERNEL_ACCOUNT);
 
 	if (pipe->bufs) {
-		init_waitqueue_head(&pipe->wait);
+		init_waitqueue_head(&pipe->rd_wait);
+		init_waitqueue_head(&pipe->wr_wait);
 		pipe->r_counter = pipe->w_counter = 1;
 		pipe->max_usage = pipe_bufs;
 		pipe->ring_size = pipe_bufs;
@@ -1007,7 +1026,8 @@ static int wait_for_partner(struct pipe_inode_info *pipe, unsigned int *cnt)
 
 static void wake_up_partner(struct pipe_inode_info *pipe)
 {
-	wake_up_interruptible(&pipe->wait);
+	wake_up_interruptible(&pipe->rd_wait);
+	wake_up_interruptible(&pipe->wr_wait);
 }
 
 static int fifo_open(struct inode *inode, struct file *filp)
@@ -1118,13 +1138,13 @@ static int fifo_open(struct inode *inode, struct file *filp)
 
 err_rd:
 	if (!--pipe->readers)
-		wake_up_interruptible(&pipe->wait);
+		wake_up_interruptible(&pipe->wr_wait);
 	ret = -ERESTARTSYS;
 	goto err;
 
 err_wr:
 	if (!--pipe->writers)
-		wake_up_interruptible(&pipe->wait);
+		wake_up_interruptible(&pipe->rd_wait);
 	ret = -ERESTARTSYS;
 	goto err;
 
@@ -1251,7 +1271,8 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 	pipe->max_usage = nr_slots;
 	pipe->tail = tail;
 	pipe->head = head;
-	wake_up_interruptible_all(&pipe->wait);
+	wake_up_interruptible_all(&pipe->rd_wait);
+	wake_up_interruptible_all(&pipe->wr_wait);
 	return pipe->max_usage * PAGE_SIZE;
 
 out_revert_acct:

fs/splice.c

@@ -165,8 +165,8 @@ static const struct pipe_buf_operations user_page_pipe_buf_ops = {
 static void wakeup_pipe_readers(struct pipe_inode_info *pipe)
 {
 	smp_mb();
-	if (waitqueue_active(&pipe->wait))
-		wake_up_interruptible(&pipe->wait);
+	if (waitqueue_active(&pipe->rd_wait))
+		wake_up_interruptible(&pipe->rd_wait);
 	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 }
 
@@ -462,8 +462,8 @@ static int pipe_to_sendpage(struct pipe_inode_info *pipe,
 static void wakeup_pipe_writers(struct pipe_inode_info *pipe)
 {
 	smp_mb();
-	if (waitqueue_active(&pipe->wait))
-		wake_up_interruptible(&pipe->wait);
+	if (waitqueue_active(&pipe->wr_wait))
+		wake_up_interruptible(&pipe->wr_wait);
 	kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 }
 

include/linux/pipe_fs_i.h

@@ -47,7 +47,7 @@ struct pipe_buffer {
  **/
 struct pipe_inode_info {
 	struct mutex mutex;
-	wait_queue_head_t wait;
+	wait_queue_head_t rd_wait, wr_wait;
 	unsigned int head;
 	unsigned int tail;
 	unsigned int max_usage;