[Qemu-devel] [PATCH 0/3] nbd: Adapt for dataplane

[Qemu-devel] [PATCH 0/3] nbd: Adapt for dataplane

[Max Reitz]

For the NBD server to work with dataplane, it needs to correctly access
the exported BDS. It makes the most sense to run both in the same
AioContext, therefore this series implements methods for tracking a
BDS's AioContext and makes NBD make use of this for keeping the clients
connected to that BDS in the same AioContext.

This series differs from the RFC as follows:
- The first patch for export_set_name() has been dropped.
- nbd_can_read() has been dropped and all attempts on implementing the
  required functionality for aio_set_fd_handler() as well. This is now
  implemented directly in NBD by unregistering the read handler whenever
  NBD cannot read.
- Renamed "AIO followers" to "AIO notifiers".

I tested this patch as Stefan suggested, that is, created IO load in the
guest with multiple dds from the block devices and then read from random
addresses over NBD (two block devices and one NBD client per device). I
didn't run into any problems and the NBD code indeed did attach and
detach from the respective AIO contexts, so this time I'm more confident
that it indeed does work.


Max Reitz (3):
  nbd: Drop nbd_can_read()
  block: Add AIO context notifiers
  nbd: Follow the BDS' AIO context

 block.c                   |  56 ++++++++++++++++++++++++
 include/block/block_int.h |  41 ++++++++++++++++++
 nbd.c                     | 108 ++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 186 insertions(+), 19 deletions(-)

-- 
2.0.0

[Qemu-devel] [PATCH 1/3] nbd: Drop nbd_can_read()

[Max Reitz]

There is no variant of aio_set_fd_handler() like qemu_set_fd_handler2(),
so we cannot give a can_read() callback function. Instead, unregister
the nbd_read() function whenever we cannot read and re-register it as
soon as we can read again.

All this is hidden behind the functions nbd_set_handlers() (which
registers all handlers for the AIO context and file descriptor belonging
to the given client), nbd_unset_handlers() (which unregisters them) and
nbd_update_can_read() (which checks whether NBD can read for the given
client and acts accordingly).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 nbd.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 58 insertions(+), 19 deletions(-)

diff --git a/nbd.c b/nbd.c
index e0d032c..c25eac7 100644
--- a/nbd.c
+++ b/nbd.c
@@ -18,6 +18,7 @@
 
 #include "block/nbd.h"
 #include "block/block.h"
+#include "block/block_int.h"
 
 #include "block/coroutine.h"
 
@@ -100,6 +101,8 @@ struct NBDExport {
     uint32_t nbdflags;
     QTAILQ_HEAD(, NBDClient) clients;
     QTAILQ_ENTRY(NBDExport) next;
+
+    AioContext *ctx;
 };
 
 static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
@@ -116,6 +119,9 @@ struct NBDClient {
     CoMutex send_lock;
     Coroutine *send_coroutine;
 
+    bool can_read;
+    bool restart_write;
+
     QTAILQ_ENTRY(NBDClient) next;
     int nb_requests;
     bool closing;
@@ -123,6 +129,10 @@ struct NBDClient {
 
 /* That's all folks */
 
+static void nbd_set_handlers(NBDClient *client);
+static void nbd_unset_handlers(NBDClient *client);
+static void nbd_update_can_read(NBDClient *client);
+
 ssize_t nbd_wr_sync(int fd, void *buffer, size_t size, bool do_read)
 {
     size_t offset = 0;
@@ -744,7 +754,7 @@ void nbd_client_put(NBDClient *client)
          */
         assert(client->closing);
 
-        qemu_set_fd_handler2(client->sock, NULL, NULL, NULL, NULL);
+        nbd_unset_handlers(client);
         close(client->sock);
         client->sock = -1;
         if (client->exp) {
@@ -780,6 +790,7 @@ static NBDRequest *nbd_request_get(NBDClient *client)
 
     assert(client->nb_requests <= MAX_NBD_REQUESTS - 1);
     client->nb_requests++;
+    nbd_update_can_read(client);
 
     req = g_slice_new0(NBDRequest);
     nbd_client_get(client);
@@ -796,9 +807,8 @@ static void nbd_request_put(NBDRequest *req)
     }
     g_slice_free(NBDRequest, req);
 
-    if (client->nb_requests-- == MAX_NBD_REQUESTS) {
-        qemu_notify_event();
-    }
+    client->nb_requests--;
+    nbd_update_can_read(client);
     nbd_client_put(client);
 }
 
@@ -814,6 +824,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset,
     exp->nbdflags = nbdflags;
     exp->size = size == -1 ? bdrv_getlength(bs) : size;
     exp->close = close;
+    exp->ctx = bdrv_get_aio_context(bs);
     bdrv_ref(bs);
     return exp;
 }
@@ -905,10 +916,6 @@ void nbd_export_close_all(void)
     }
 }
 
-static int nbd_can_read(void *opaque);
-static void nbd_read(void *opaque);
-static void nbd_restart_write(void *opaque);
-
 static ssize_t nbd_co_send_reply(NBDRequest *req, struct nbd_reply *reply,
                                  int len)
 {
@@ -917,8 +924,8 @@ static ssize_t nbd_co_send_reply(NBDRequest *req, struct nbd_reply *reply,
     ssize_t rc, ret;
 
     qemu_co_mutex_lock(&client->send_lock);
-    qemu_set_fd_handler2(csock, nbd_can_read, nbd_read,
-                         nbd_restart_write, client);
+    client->restart_write = true;
+    nbd_set_handlers(client);
     client->send_coroutine = qemu_coroutine_self();
 
     if (!len) {
@@ -936,7 +943,8 @@ static ssize_t nbd_co_send_reply(NBDRequest *req, struct nbd_reply *reply,
     }
 
     client->send_coroutine = NULL;
-    qemu_set_fd_handler2(csock, nbd_can_read, nbd_read, NULL, client);
+    client->restart_write = false;
+    nbd_set_handlers(client);
     qemu_co_mutex_unlock(&client->send_lock);
     return rc;
 }
@@ -949,6 +957,8 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, struct nbd_request *reque
     ssize_t rc;
 
     client->recv_coroutine = qemu_coroutine_self();
+    nbd_update_can_read(client);
+
     rc = nbd_receive_request(csock, request);
     if (rc < 0) {
         if (rc != -EAGAIN) {
@@ -990,6 +1000,8 @@ static ssize_t nbd_co_receive_request(NBDRequest *req, struct nbd_request *reque
 
 out:
     client->recv_coroutine = NULL;
+    nbd_update_can_read(client);
+
     return rc;
 }
 
@@ -1140,13 +1152,6 @@ out:
     nbd_client_close(client);
 }
 
-static int nbd_can_read(void *opaque)
-{
-    NBDClient *client = opaque;
-
-    return client->recv_coroutine || client->nb_requests < MAX_NBD_REQUESTS;
-}
-
 static void nbd_read(void *opaque)
 {
     NBDClient *client = opaque;
@@ -1165,6 +1170,39 @@ static void nbd_restart_write(void *opaque)
     qemu_coroutine_enter(client->send_coroutine, NULL);
 }
 
+static void nbd_set_handlers(NBDClient *client)
+{
+    if (client->exp && client->exp->ctx) {
+        aio_set_fd_handler(client->exp->ctx, client->sock,
+                           client->can_read ? nbd_read : NULL,
+                           client->restart_write ? nbd_restart_write : NULL,
+                           client);
+    }
+}
+
+static void nbd_unset_handlers(NBDClient *client)
+{
+    if (client->exp && client->exp->ctx) {
+        aio_set_fd_handler(client->exp->ctx, client->sock, NULL, NULL, NULL);
+    }
+}
+
+static void nbd_update_can_read(NBDClient *client)
+{
+    bool can_read = client->recv_coroutine ||
+                    client->nb_requests < MAX_NBD_REQUESTS;
+
+    if (can_read != client->can_read) {
+        client->can_read = can_read;
+        nbd_set_handlers(client);
+
+        /* If we got here, nb_requests had to be MAX_NBD_REQUESTS before */
+        if (client->nb_requests < MAX_NBD_REQUESTS) {
+            aio_notify(client->exp->ctx);
+        }
+    }
+}
+
 NBDClient *nbd_client_new(NBDExport *exp, int csock,
                           void (*close)(NBDClient *))
 {
@@ -1173,13 +1211,14 @@ NBDClient *nbd_client_new(NBDExport *exp, int csock,
     client->refcount = 1;
     client->exp = exp;
     client->sock = csock;
+    client->can_read = true;
     if (nbd_send_negotiate(client) < 0) {
         g_free(client);
         return NULL;
     }
     client->close = close;
     qemu_co_mutex_init(&client->send_lock);
-    qemu_set_fd_handler2(csock, nbd_can_read, nbd_read, NULL, client);
+    nbd_set_handlers(client);
 
     if (exp) {
         QTAILQ_INSERT_TAIL(&exp->clients, client, next);
-- 
2.0.0

Re: [Qemu-devel] [PATCH 1/3] nbd: Drop nbd_can_read()

[Paolo Bonzini]

Il 14/06/2014 00:30, Max Reitz ha scritto:
> +static void nbd_set_handlers(NBDClient *client)
> +{
> +    if (client->exp && client->exp->ctx) {
> +        aio_set_fd_handler(client->exp->ctx, client->sock,
> +                           client->can_read ? nbd_read : NULL,
> +                           client->restart_write ? nbd_restart_write : NULL,

Instead of client->restart_write you can just use 
client->send_coroutine.  Apart from this, the patch is okay.

Paolo

> +                           client);
> +    }
> +}
> +

Re: [Qemu-devel] [PATCH 1/3] nbd: Drop nbd_can_read()

[Max Reitz]

On 14.06.2014 09:57, Paolo Bonzini wrote:
> Il 14/06/2014 00:30, Max Reitz ha scritto:
>> +static void nbd_set_handlers(NBDClient *client)
>> +{
>> +    if (client->exp && client->exp->ctx) {
>> +        aio_set_fd_handler(client->exp->ctx, client->sock,
>> +                           client->can_read ? nbd_read : NULL,
>> +                           client->restart_write ? nbd_restart_write 
>> : NULL,
>
> Instead of client->restart_write you can just use 
> client->send_coroutine.  Apart from this, the patch is okay.

Oh, right, thanks.

Max

[Qemu-devel] [PATCH 2/3] block: Add AIO context notifiers

[Max Reitz]

If a long-running operation on a BDS wants to always remain in the same
AIO context, it somehow needs to keep track of the BDS changing its
context. This adds a function for registering callbacks on a BDS which
are called whenever the BDS is attached or detached from an AIO context.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c                   | 56 +++++++++++++++++++++++++++++++++++++++++++++++
 include/block/block_int.h | 41 ++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+)

diff --git a/block.c b/block.c
index 17f763d..1ee82cc 100644
--- a/block.c
+++ b/block.c
@@ -1809,6 +1809,8 @@ void bdrv_reopen_abort(BDRVReopenState *reopen_state)
 
 void bdrv_close(BlockDriverState *bs)
 {
+    BdrvAioNotifier *ban, *ban_next;
+
     if (bs->job) {
         block_job_cancel_sync(bs->job);
     }
@@ -1851,6 +1853,11 @@ void bdrv_close(BlockDriverState *bs)
     if (bs->io_limits_enabled) {
         bdrv_io_limits_disable(bs);
     }
+
+    QLIST_FOREACH_SAFE(ban, &bs->aio_notifiers, list, ban_next) {
+        g_free(ban);
+    }
+    QLIST_INIT(&bs->aio_notifiers);
 }
 
 void bdrv_close_all(void)
@@ -5668,10 +5675,16 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs)
 
 void bdrv_detach_aio_context(BlockDriverState *bs)
 {
+    BdrvAioNotifier *baf;
+
     if (!bs->drv) {
         return;
     }
 
+    QLIST_FOREACH(baf, &bs->aio_notifiers, list) {
+        baf->detach_aio_context(baf->opaque);
+    }
+
     if (bs->io_limits_enabled) {
         throttle_detach_aio_context(&bs->throttle_state);
     }
@@ -5691,6 +5704,8 @@ void bdrv_detach_aio_context(BlockDriverState *bs)
 void bdrv_attach_aio_context(BlockDriverState *bs,
                              AioContext *new_context)
 {
+    BdrvAioNotifier *ban;
+
     if (!bs->drv) {
         return;
     }
@@ -5709,6 +5724,10 @@ void bdrv_attach_aio_context(BlockDriverState *bs,
     if (bs->io_limits_enabled) {
         throttle_attach_aio_context(&bs->throttle_state, new_context);
     }
+
+    QLIST_FOREACH(ban, &bs->aio_notifiers, list) {
+        ban->attached_aio_context(new_context, ban->opaque);
+    }
 }
 
 void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
@@ -5725,6 +5744,43 @@ void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
     aio_context_release(new_context);
 }
 
+void bdrv_add_aio_context_notifier(BlockDriverState *bs,
+        void (*attached_aio_context)(AioContext *new_context, void *opaque),
+        void (*detach_aio_context)(void *opaque), void *opaque)
+{
+    BdrvAioNotifier *ban = g_new(BdrvAioNotifier, 1);
+    *ban = (BdrvAioNotifier){
+        .attached_aio_context = attached_aio_context,
+        .detach_aio_context   = detach_aio_context,
+        .opaque               = opaque
+    };
+
+    QLIST_INSERT_HEAD(&bs->aio_notifiers, ban, list);
+}
+
+void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
+                                      void (*attached_aio_context)(AioContext *,
+                                                                   void *),
+                                      void (*detach_aio_context)(void *),
+                                      void *opaque)
+{
+    BdrvAioNotifier *ban, *ban_next;
+
+    QLIST_FOREACH_SAFE(ban, &bs->aio_notifiers, list, ban_next) {
+        if (ban->attached_aio_context == attached_aio_context &&
+            ban->detach_aio_context   == detach_aio_context   &&
+            ban->opaque               == opaque)
+        {
+            QLIST_REMOVE(ban, list);
+            g_free(ban);
+
+            return;
+        }
+    }
+
+    abort();
+}
+
 void bdrv_add_before_write_notifier(BlockDriverState *bs,
                                     NotifierWithReturn *notifier)
 {
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8d58334..cdfc1bb 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -285,6 +285,15 @@ typedef struct BlockLimits {
 
 typedef struct BdrvOpBlocker BdrvOpBlocker;
 
+typedef struct BdrvAioNotifier {
+    void (*attached_aio_context)(AioContext *new_context, void *opaque);
+    void (*detach_aio_context)(void *opaque);
+
+    void *opaque;
+
+    QLIST_ENTRY(BdrvAioNotifier) list;
+} BdrvAioNotifier;
+
 /*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
@@ -311,6 +320,10 @@ struct BlockDriverState {
     void *dev_opaque;
 
     AioContext *aio_context; /* event loop used for fd handlers, timers, etc */
+    /* long-running tasks intended to always use the same AioContext as this
+     * BDS may register themselves in this list to be notified of changes
+     * regarding this BDS's context */
+    QLIST_HEAD(, BdrvAioNotifier) aio_notifiers;
 
     char filename[1024];
     char backing_file[1024]; /* if non zero, the image is a diff of
@@ -425,6 +438,34 @@ void bdrv_detach_aio_context(BlockDriverState *bs);
 void bdrv_attach_aio_context(BlockDriverState *bs,
                              AioContext *new_context);
 
+/**
+ * bdrv_add_aio_context_notifier:
+ *
+ * If a long-running job intends to be always run in the same AioContext as a
+ * certain BDS, it may use this function to be notified of changes regarding the
+ * association of the BDS to an AioContext.
+ *
+ * attached_aio_context() is called after the target BDS has been attached to a
+ * new AioContext; detach_aio_context() is called before the target BDS is being
+ * detached from its old AioContext.
+ */
+void bdrv_add_aio_context_notifier(BlockDriverState *bs,
+        void (*attached_aio_context)(AioContext *new_context, void *opaque),
+        void (*detach_aio_context)(void *opaque), void *opaque);
+
+/**
+ * bdrv_remove_aio_context_notifier:
+ *
+ * Unsubscribe of change notifications regarding the BDS's AioContext. The
+ * parameters given here have to be the same as those given to
+ * bdrv_add_aio_context_notifier().
+ */
+void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
+                                      void (*aio_context_attached)(AioContext *,
+                                                                   void *),
+                                      void (*aio_context_detached)(void *),
+                                      void *opaque);
+
 #ifdef _WIN32
 int is_windows_drive(const char *filename);
 #endif
-- 
2.0.0

[Qemu-devel] [PATCH 3/3] nbd: Follow the BDS' AIO context

[Max Reitz]

Keep the NBD server always in the same AIO context as the exported BDS
by calling bdrv_add_aio_context_notifier() and implementing the required
callbacks.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 nbd.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/nbd.c b/nbd.c
index c25eac7..ae466da 100644
--- a/nbd.c
+++ b/nbd.c
@@ -812,6 +812,34 @@ static void nbd_request_put(NBDRequest *req)
     nbd_client_put(client);
 }
 
+static void bs_aio_attached(AioContext *ctx, void *opaque)
+{
+    NBDExport *exp = opaque;
+    NBDClient *client;
+
+    TRACE("Export %s: Attaching clients to AIO context %p\n", exp->name, ctx);
+
+    exp->ctx = ctx;
+
+    QTAILQ_FOREACH(client, &exp->clients, next) {
+        nbd_set_handlers(client);
+    }
+}
+
+static void bs_aio_detach(void *opaque)
+{
+    NBDExport *exp = opaque;
+    NBDClient *client;
+
+    TRACE("Export %s: Detaching clients from AIO context %p\n", exp->name, exp->ctx);
+
+    QTAILQ_FOREACH(client, &exp->clients, next) {
+        nbd_unset_handlers(client);
+    }
+
+    exp->ctx = NULL;
+}
+
 NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset,
                           off_t size, uint32_t nbdflags,
                           void (*close)(NBDExport *))
@@ -826,6 +854,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset,
     exp->close = close;
     exp->ctx = bdrv_get_aio_context(bs);
     bdrv_ref(bs);
+    bdrv_add_aio_context_notifier(bs, bs_aio_attached, bs_aio_detach, exp);
     return exp;
 }
 
@@ -873,6 +902,8 @@ void nbd_export_close(NBDExport *exp)
     nbd_export_set_name(exp, NULL);
     nbd_export_put(exp);
     if (exp->bs) {
+        bdrv_remove_aio_context_notifier(exp->bs, bs_aio_attached,
+                                         bs_aio_detach, exp);
         bdrv_unref(exp->bs);
         exp->bs = NULL;
     }
-- 
2.0.0