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io_uring: limit parallelism of buffered writes

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All the popular filesystems need to grab the inode lock for buffered
writes. With io_uring punting buffered writes to async context, we
observe a lot of contention with all workers hamming this mutex.

For buffered writes, we generally don't need a lot of parallelism on
the submission side, as the flushing will take care of that for us.
Hence we don't need a deep queue on the write side, as long as we
can safely punt from the original submission context.

Add a workqueue with a limit of 2 that we can use for buffered writes.
This greatly improves the performance and efficiency of higher queue
depth buffered async writes with io_uring.

Reported-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2019-09-10 09:15:04 -06:00
parent 18d9be1a97
commit 54a91f3bb9
1 changed files with 39 additions and 8 deletions
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47
fs/io_uring.c
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@ -203,7 +203,7 @@ struct io_ring_ctx {
} ____cacheline_aligned_in_smp;
/* IO offload */
struct workqueue_struct *sqo_wq;
struct workqueue_struct *sqo_wq[2];
struct task_struct *sqo_thread; /* if using sq thread polling */
struct mm_struct *sqo_mm;
wait_queue_head_t sqo_wait;
@ -446,7 +446,19 @@ static void __io_commit_cqring(struct io_ring_ctx *ctx)
static inline void io_queue_async_work(struct io_ring_ctx *ctx,
struct io_kiocb *req)
{
queue_work(ctx->sqo_wq, &req->work);
int rw;
switch (req->submit.sqe->opcode) {
case IORING_OP_WRITEV:
case IORING_OP_WRITE_FIXED:
rw = !(req->rw.ki_flags & IOCB_DIRECT);
break;
default:
rw = 0;
break;
}
queue_work(ctx->sqo_wq[rw], &req->work);
}
static void io_commit_cqring(struct io_ring_ctx *ctx)
@ -2634,11 +2646,15 @@ static void io_sq_thread_stop(struct io_ring_ctx *ctx)
static void io_finish_async(struct io_ring_ctx *ctx)
{
int i;
io_sq_thread_stop(ctx);
if (ctx->sqo_wq) {
destroy_workqueue(ctx->sqo_wq);
ctx->sqo_wq = NULL;
for (i = 0; i < ARRAY_SIZE(ctx->sqo_wq); i++) {
if (ctx->sqo_wq[i]) {
destroy_workqueue(ctx->sqo_wq[i]);
ctx->sqo_wq[i] = NULL;
}
}
}
@ -2846,16 +2862,31 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
}
/* Do QD, or 2 * CPUS, whatever is smallest */
ctx->sqo_wq = alloc_workqueue("io_ring-wq", WQ_UNBOUND | WQ_FREEZABLE,
ctx->sqo_wq[0] = alloc_workqueue("io_ring-wq",
WQ_UNBOUND | WQ_FREEZABLE,
min(ctx->sq_entries - 1, 2 * num_online_cpus()));
if (!ctx->sqo_wq) {
if (!ctx->sqo_wq[0]) {
ret = -ENOMEM;
goto err;
}
/*
* This is for buffered writes, where we want to limit the parallelism
* due to file locking in file systems. As "normal" buffered writes
* should parellelize on writeout quite nicely, limit us to having 2
* pending. This avoids massive contention on the inode when doing
* buffered async writes.
*/
ctx->sqo_wq[1] = alloc_workqueue("io_ring-write-wq",
WQ_UNBOUND | WQ_FREEZABLE, 2);
if (!ctx->sqo_wq[1]) {
ret = -ENOMEM;
goto err;
}
return 0;
err:
io_sq_thread_stop(ctx);
io_finish_async(ctx);
mmdrop(ctx->sqo_mm);
ctx->sqo_mm = NULL;
return ret;