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Message-ID: <Pine.LNX.4.64.1101030956460.24679@cobra.newdream.net>
Date: Mon, 3 Jan 2011 09:58:22 -0800 (PST)
From: Sage Weil <sage@...dream.net>
To: Tejun Heo <tj@...nel.org>
cc: linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org
Subject: Re: [PATCH 23/32] net/ceph: make ceph_msgr_wq non-reentrant
On Mon, 3 Jan 2011, Tejun Heo wrote:
> ceph messenger code does a rather complex dancing around multithread
> workqueue to make sure the same work item isn't executed concurrently
> on different CPUs. This restriction can be provided by workqueue with
> WQ_NON_REENTRANT.
>
> Make ceph_msgr_wq non-reentrant workqueue with the default concurrency
> level and remove the QUEUED/BUSY logic.
>
> * This removes backoff handling in con_work() but it couldn't reliably
> block execution of con_work() to begin with - queue_con() can be
> called after the work started but before BUSY is set. It seems that
> it was an optimization for a rather cold path and can be safely
> removed.
>
> * The number of concurrent work items is bound by the number of
> connections and connetions are independent from each other. With
> the default concurrency level, different connections will be
> executed independently.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Sage Weil <sage@...dream.net>
> Cc: ceph-devel@...r.kernel.org
> ---
> Only compile tested. I think the dropping of backoff logic is safe
> but am not completely sure. Please verify it's actually okay. Feel
> free to take it into the subsystem tree or simply ack - I'll route it
> through the wq tree.
Tejun- this is a very welcome simplification! I'll take it through the
ceph tree, as I want to test it thoroughly and make sure the backoff
change is correct (there was some subtlety there, IIRC).
Thanks!
sage
>
> Thanks.
>
> include/linux/ceph/messenger.h | 5 ----
> net/ceph/messenger.c | 46 +--------------------------------------
> 2 files changed, 2 insertions(+), 49 deletions(-)
>
> diff --git a/include/linux/ceph/messenger.h b/include/linux/ceph/messenger.h
> index a108b42..c3011be 100644
> --- a/include/linux/ceph/messenger.h
> +++ b/include/linux/ceph/messenger.h
> @@ -110,17 +110,12 @@ struct ceph_msg_pos {
>
> /*
> * ceph_connection state bit flags
> - *
> - * QUEUED and BUSY are used together to ensure that only a single
> - * thread is currently opening, reading or writing data to the socket.
> */
> #define LOSSYTX 0 /* we can close channel or drop messages on errors */
> #define CONNECTING 1
> #define NEGOTIATING 2
> #define KEEPALIVE_PENDING 3
> #define WRITE_PENDING 4 /* we have data ready to send */
> -#define QUEUED 5 /* there is work queued on this connection */
> -#define BUSY 6 /* work is being done */
> #define STANDBY 8 /* no outgoing messages, socket closed. we keep
> * the ceph_connection around to maintain shared
> * state with the peer. */
> diff --git a/net/ceph/messenger.c b/net/ceph/messenger.c
> index b6ff4a1..dff633d 100644
> --- a/net/ceph/messenger.c
> +++ b/net/ceph/messenger.c
> @@ -96,7 +96,7 @@ struct workqueue_struct *ceph_msgr_wq;
>
> int ceph_msgr_init(void)
> {
> - ceph_msgr_wq = create_workqueue("ceph-msgr");
> + ceph_msgr_wq = alloc_workqueue("ceph-msgr", WQ_NON_REENTRANT, 0);
> if (!ceph_msgr_wq) {
> pr_err("msgr_init failed to create workqueue\n");
> return -ENOMEM;
> @@ -1920,20 +1920,6 @@ bad_tag:
> /*
> * Atomically queue work on a connection. Bump @con reference to
> * avoid races with connection teardown.
> - *
> - * There is some trickery going on with QUEUED and BUSY because we
> - * only want a _single_ thread operating on each connection at any
> - * point in time, but we want to use all available CPUs.
> - *
> - * The worker thread only proceeds if it can atomically set BUSY. It
> - * clears QUEUED and does it's thing. When it thinks it's done, it
> - * clears BUSY, then rechecks QUEUED.. if it's set again, it loops
> - * (tries again to set BUSY).
> - *
> - * To queue work, we first set QUEUED, _then_ if BUSY isn't set, we
> - * try to queue work. If that fails (work is already queued, or BUSY)
> - * we give up (work also already being done or is queued) but leave QUEUED
> - * set so that the worker thread will loop if necessary.
> */
> static void queue_con(struct ceph_connection *con)
> {
> @@ -1948,11 +1934,7 @@ static void queue_con(struct ceph_connection *con)
> return;
> }
>
> - set_bit(QUEUED, &con->state);
> - if (test_bit(BUSY, &con->state)) {
> - dout("queue_con %p - already BUSY\n", con);
> - con->ops->put(con);
> - } else if (!queue_work(ceph_msgr_wq, &con->work.work)) {
> + if (!queue_delayed_work(ceph_msgr_wq, &con->work, 0)) {
> dout("queue_con %p - already queued\n", con);
> con->ops->put(con);
> } else {
> @@ -1967,15 +1949,6 @@ static void con_work(struct work_struct *work)
> {
> struct ceph_connection *con = container_of(work, struct ceph_connection,
> work.work);
> - int backoff = 0;
> -
> -more:
> - if (test_and_set_bit(BUSY, &con->state) != 0) {
> - dout("con_work %p BUSY already set\n", con);
> - goto out;
> - }
> - dout("con_work %p start, clearing QUEUED\n", con);
> - clear_bit(QUEUED, &con->state);
>
> mutex_lock(&con->mutex);
>
> @@ -1994,28 +1967,13 @@ more:
> try_read(con) < 0 ||
> try_write(con) < 0) {
> mutex_unlock(&con->mutex);
> - backoff = 1;
> ceph_fault(con); /* error/fault path */
> goto done_unlocked;
> }
>
> done:
> mutex_unlock(&con->mutex);
> -
> done_unlocked:
> - clear_bit(BUSY, &con->state);
> - dout("con->state=%lu\n", con->state);
> - if (test_bit(QUEUED, &con->state)) {
> - if (!backoff || test_bit(OPENING, &con->state)) {
> - dout("con_work %p QUEUED reset, looping\n", con);
> - goto more;
> - }
> - dout("con_work %p QUEUED reset, but just faulted\n", con);
> - clear_bit(QUEUED, &con->state);
> - }
> - dout("con_work %p done\n", con);
> -
> -out:
> con->ops->put(con);
> }
>
> --
> 1.7.1
>
> --
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