From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit 0ddad21d3e99c743a3aa473121dc5561679e26bb ]
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 josh@joshtriplett.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/coredump.c | 4 +-- fs/pipe.c | 67 +++++++++++++++++++++++++-------------- fs/splice.c | 8 ++--- include/linux/pipe_fs_i.h | 2 +- 4 files changed, 51 insertions(+), 30 deletions(-)
diff --git a/fs/coredump.c b/fs/coredump.c index b1ea7dfbd1494..f8296a82d01df 100644 --- a/fs/coredump.c +++ b/fs/coredump.c @@ -517,7 +517,7 @@ 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);
@@ -525,7 +525,7 @@ static void wait_for_dump_helpers(struct file *file) * 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--; diff --git a/fs/pipe.c b/fs/pipe.c index 57502c3c0fba1..5a34d6c22d4ce 100644 --- a/fs/pipe.c +++ b/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: diff --git a/fs/splice.c b/fs/splice.c index 3009652a41c85..d671936d0aad6 100644 --- a/fs/splice.c +++ b/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); }
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h index dbcfa68923842..d5765039652a5 100644 --- a/include/linux/pipe_fs_i.h +++ b/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;