Re: [PATCH] io_uring: Try to merge io requests only for regular files

[Dmitry Monakhov]

- stable@



19.03.2021, 08:29, "Dmitry Monakhov" <dmtrmonakhov@xxxxxxxxxxxxxx>:
> Otherwise we may endup blocking on pipe or socket.
>
> Fixes: 6d5d5ac ("io_uring: extend async work merge")
> Testcase: https://github.com/dmonakhov/liburing/commit/16d171b6ef9d68e6db66650a83d98c5c721d01f6
> Signed-off-by: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx>
> ---
>  fs/io_uring.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 478df7e..848657c 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2183,6 +2183,9 @@ static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>  static struct async_list *io_async_list_from_req(struct io_ring_ctx *ctx,
>                                                   struct io_kiocb *req)
>  {
> + if (!(req->flags & REQ_F_ISREG))
> + return NULL;
> +
IMHO it is reasonable to completely disable  io_should_merge logic because
even with the this fix it still affected by latency spikes like follows:

->submit_read: req1[slow_hdd, sector=z]
->submit_read: req2[nvme, sector=X]
-> wait(req2)  -> fast

->submit_read: req3[nvme, sector=Y] 
--> wait(req3)  ->slow  
  if completes if X and Y belongs to same page merge logic will wait for req1 to complete



From c24cda9aa902775e4e23575b758f009e9fae4640 Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx>
Date: Fri, 19 Mar 2021 08:48:18 +0300
Subject: [PATCH] io_uring: completely disable io_should_merge logic

io_should_merge logic is wierd and prone to latency spikes because of slow neighbors.

Signed-off-by: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 478df7e..a4fb3a7 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1288,7 +1288,7 @@ static ssize_t io_import_iovec(struct io_ring_ctx *ctx, int rw,
 
 static inline bool io_should_merge(struct async_list *al, struct kiocb *kiocb)
 {
-	if (al->file == kiocb->ki_filp) {
+	if (al->file == kiocb->ki_filp && 0) {
 		off_t start, end;
 
 		/*
-- 
2.7.4