Re: [PATCH] Relay CPU Hotplug support

From: Tom Zanussi
Date: Thu Dec 21 2006 - 05:38:15 EST

Andrew Morton writes:
> On Wed, 20 Dec 2006 19:31:01 -0500
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
>
> > Hi,
> >
> > Here is a patch, result of the combined work of Tom Zanussi and myself, to add
> > CPU hotplug support to Relay.
> >
> > ...
> >
> > +
> > + lock_cpu_hotplug();
> > + for_each_online_cpu(i)
> > + if (chan->buf[i])
> > + __relay_reset(chan->buf[i], 0);
> > + unlock_cpu_hotplug();
>
> __relay_reset() runs flush_scheduled_work(). If one of the queued works
> takes lock_cpu_hoplug() (and some do), thou art most deadest meat.
>
> I think the core design problem you have here is that you are using
> cpu_online_map to record the presence or absence of resources which belong
> to the relay driver. Why do that - you don't own cpu_online_map (but you
> do get some notifications when it wants to change, that's all).
>
> Perhaps a better approach would be to teach the relay driver to maintain
> its own resources (already there, in chan->buf[]). So relay.c has a record
> of which per-cpu buffers are present and which are not. That information
> gets changed under a lock which the relay driver owns and controls.
>
> You already have such a lock: relay_channels_mutex. So some code in here
> is using lock_cpu_hotplug() to protect relay's resources while other code
> is using relay_channels_mutex. Which is it?
>

Yes, I think we should be able to get rid of lock_cpu_hotplug() and
just use relay_channels_mutex instead i.e., we can't be adding a
buffer for a new cpu if we're iterating the cpu_online_map and vice
versa; the below patch makes that change.

>
> Your proposed change apparently chooses to not release per-cpu resources on
> cpu-hotunplug. I think. That's the sort of thing which should be
> communicated in the (presently non-existent) patch changelog.
>

Yes, you're right - unplug isn't supported by this patch. The thought
was that at minumum a new buffer needs to be added when a cpu comes
up, but it wasn't worth the effort to remove buffers on cpu down since
they'd be freed soon anyway when the channel was closed.

> The changelog should also tell us *why* this patch was written. Right now
> it's in "why on earth should we merge this" territory.
>

Mathieu originally needed to add this for tracing Xen, but it's
something that's needed for any application that can be tracing while
cpus are added.


Tom

diff --git a/Documentation/filesystems/relay.txt b/Documentation/filesystems/relay.txt
index d6788da..7fbb6ff 100644
--- a/Documentation/filesystems/relay.txt
+++ b/Documentation/filesystems/relay.txt
@@ -157,7 +157,7 @@ TBD(curr. line MT:/API/)
channel management functions:

relay_open(base_filename, parent, subbuf_size, n_subbufs,
- callbacks)
+ callbacks, private_data)
relay_close(chan)
relay_flush(chan)
relay_reset(chan)
@@ -251,7 +251,7 @@ static struct rchan_callbacks relay_call

And an example relay_open() invocation using them:

- chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks);
+ chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks, NULL);

If the create_buf_file() callback fails, or isn't defined, channel
creation and thus relay_open() will fail.
@@ -289,6 +289,11 @@ they use the proper locking for such a b
writes in a spinlock, or by copying a write function from relay.h and
creating a local version that internally does the proper locking.

+The private_data passed into relay_open() allows clients to associate
+user-defined data with a channel, and is immediately available
+(including in create_buf_file()) via chan->private_data or
+buf->chan->private_data.
+
Channel 'modes'
---------------

diff --git a/block/blktrace.c b/block/blktrace.c
index 2a248ec..1230354 100644
--- a/block/blktrace.c
+++ b/block/blktrace.c
@@ -335,10 +335,9 @@ static int blk_trace_setup(request_queue
if (!bt->dropped_file)
goto err;

- bt->rchan = relay_open("trace", dir, buts.buf_size, buts.buf_nr, &blk_relay_callbacks);
+ bt->rchan = relay_open("trace", dir, buts.buf_size, buts.buf_nr, &blk_relay_callbacks, bt);
if (!bt->rchan)
goto err;
- bt->rchan->private_data = bt;

bt->act_mask = buts.act_mask;
if (!bt->act_mask)
diff --git a/include/linux/relay.h b/include/linux/relay.h
index 24accb4..c275e89 100644
--- a/include/linux/relay.h
+++ b/include/linux/relay.h
@@ -24,7 +24,7 @@
/*
* Tracks changes to rchan/rchan_buf structs
*/
-#define RELAYFS_CHANNEL_VERSION 6
+#define RELAYFS_CHANNEL_VERSION 7

/*
* Per-cpu relay channel buffer
@@ -64,6 +64,10 @@ struct rchan
void *private_data; /* for user-defined data */
size_t last_toobig; /* tried to log event > subbuf size */
struct rchan_buf *buf[NR_CPUS]; /* per-cpu channel buffers */
+ int is_global; /* One global buffer ? */
+ struct list_head list; /* for channel list */
+ struct dentry *parent; /* parent dentry passed to open */
+ char base_filename[NAME_MAX]; /* saved base filename */
};

/*
@@ -162,7 +166,8 @@ struct rchan *relay_open(const char *bas
struct dentry *parent,
size_t subbuf_size,
size_t n_subbufs,
- struct rchan_callbacks *cb);
+ struct rchan_callbacks *cb,
+ void *private_data);
extern void relay_close(struct rchan *chan);
extern void relay_flush(struct rchan *chan);
extern void relay_subbufs_consumed(struct rchan *chan,
diff --git a/kernel/relay.c b/kernel/relay.c
index f04bbdb..df4de52 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -7,6 +7,8 @@
* Copyright (C) 1999-2005 - Karim Yaghmour (karim@xxxxxxxxxxx)
*
* Moved to kernel/relay.c by Paul Mundt, 2006.
+ * November 2006 - CPU hotplug support by Mathieu Desnoyers
+ * (mathieu.desnoyers@xxxxxxxxxx)
*
* This file is released under the GPL.
*/
@@ -18,6 +20,11 @@
#include <linux/relay.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
+#include <linux/cpu.h>
+
+/* list of open channels, for cpu hotplug */
+static DEFINE_MUTEX(relay_channels_mutex);
+static LIST_HEAD(relay_channels);

/*
* close() vm_op implementation for relay file mapping.
@@ -187,6 +194,7 @@ void relay_destroy_buf(struct rchan_buf
__free_page(buf->page_array[i]);
kfree(buf->page_array);
}
+ chan->buf[buf->cpu] = NULL;
kfree(buf->padding);
kfree(buf);
kref_put(&chan->kref, relay_destroy_channel);
@@ -361,58 +369,76 @@ static inline void __relay_reset(struct
void relay_reset(struct rchan *chan)
{
unsigned int i;
- struct rchan_buf *prev = NULL;

if (!chan)
return;

- for (i = 0; i < NR_CPUS; i++) {
- if (!chan->buf[i] || chan->buf[i] == prev)
- break;
- __relay_reset(chan->buf[i], 0);
- prev = chan->buf[i];
+ if (chan->is_global && chan->buf[0]) {
+ __relay_reset(chan->buf[0], 0);
+ return;
}
+
+ mutex_lock(&relay_channels_mutex);
+ for_each_online_cpu(i)
+ if (chan->buf[i])
+ __relay_reset(chan->buf[i], 0);
+ mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_reset);

/*
* relay_open_buf - create a new relay channel buffer
*
- * Internal - used by relay_open().
+ * used by relay_open() and CPU hotplug.
*/
-static struct rchan_buf *relay_open_buf(struct rchan *chan,
- const char *filename,
- struct dentry *parent,
- int *is_global)
+static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
{
- struct rchan_buf *buf;
+ struct rchan_buf *buf = NULL;
struct dentry *dentry;
+ char *tmpname;

- if (*is_global)
+ if (chan->is_global)
return chan->buf[0];

+ tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
+ if (!tmpname)
+ goto end;
+ snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);
+
buf = relay_create_buf(chan);
if (!buf)
- return NULL;
+ goto free_name;
+
+ buf->cpu = cpu;
+ __relay_reset(buf, 1);

/* Create file in fs */
- dentry = chan->cb->create_buf_file(filename, parent, S_IRUSR,
- buf, is_global);
- if (!dentry) {
- relay_destroy_buf(buf);
- return NULL;
- }
+ dentry = chan->cb->create_buf_file(tmpname, chan->parent, S_IRUSR,
+ buf, &chan->is_global);
+ if (!dentry)
+ goto free_buf;

buf->dentry = dentry;
- __relay_reset(buf, 1);

+ if(chan->is_global) {
+ chan->buf[0] = buf;
+ buf->cpu = 0;
+ }
+
+ goto free_name;
+
+free_buf:
+ relay_destroy_buf(buf);
+free_name:
+ kfree(tmpname);
+end:
return buf;
}

/**
* relay_close_buf - close a channel buffer
* @buf: channel buffer
- *
+ *
* Marks the buffer finalized and restores the default callbacks.
* The channel buffer and channel buffer data structure are then freed
* automatically when the last reference is given up.
@@ -447,12 +473,54 @@ static inline void setup_callbacks(struc
}

/**
+ *
+ * relay_hotcpu_callback - CPU hotplug callback
+ * @nb: notifier block
+ * @action: hotplug action to take
+ * @hcpu: CPU number
+ *
+ * Returns the success/failure of the operation. (NOTIFY_OK, NOTIFY_BAD)
+ */
+static int __cpuinit relay_hotcpu_callback(struct notifier_block *nb,
+ unsigned long action,
+ void *hcpu)
+{
+ unsigned int hotcpu = (unsigned long)hcpu;
+ struct rchan *chan;
+
+ switch(action) {
+ case CPU_UP_PREPARE:
+ mutex_lock(&relay_channels_mutex);
+ list_for_each_entry(chan, &relay_channels, list) {
+ if (chan->buf[hotcpu])
+ continue;
+ chan->buf[hotcpu] = relay_open_buf(chan, hotcpu);
+ if(!chan->buf[hotcpu]) {
+ printk(KERN_ERR
+ "relay_hotcpu_callback: cpu %d buffer "
+ "creation failed\n", hotcpu);
+ mutex_unlock(&relay_channels_mutex);
+ return NOTIFY_BAD;
+ }
+ }
+ mutex_unlock(&relay_channels_mutex);
+ break;
+ case CPU_DEAD:
+ /* No need to flush the cpu : will be flushed upon
+ * final relay_flush() call. */
+ break;
+ }
+ return NOTIFY_OK;
+}
+
+/**
* relay_open - create a new relay channel
* @base_filename: base name of files to create
* @parent: dentry of parent directory, %NULL for root directory
* @subbuf_size: size of sub-buffers
* @n_subbufs: number of sub-buffers
* @cb: client callback functions
+ * @private_data: user-defined data
*
* Returns channel pointer if successful, %NULL otherwise.
*
@@ -465,13 +533,11 @@ struct rchan *relay_open(const char *bas
struct dentry *parent,
size_t subbuf_size,
size_t n_subbufs,
- struct rchan_callbacks *cb)
+ struct rchan_callbacks *cb,
+ void *private_data)
{
unsigned int i;
struct rchan *chan;
- char *tmpname;
- int is_global = 0;
-
if (!base_filename)
return NULL;

@@ -486,38 +552,32 @@ struct rchan *relay_open(const char *bas
chan->n_subbufs = n_subbufs;
chan->subbuf_size = subbuf_size;
chan->alloc_size = FIX_SIZE(subbuf_size * n_subbufs);
+ chan->parent = parent;
+ chan->private_data = private_data;
+ strlcpy(chan->base_filename, base_filename, NAME_MAX);
setup_callbacks(chan, cb);
kref_init(&chan->kref);

- tmpname = kmalloc(NAME_MAX + 1, GFP_KERNEL);
- if (!tmpname)
- goto free_chan;
-
+ mutex_lock(&relay_channels_mutex);
for_each_online_cpu(i) {
- sprintf(tmpname, "%s%d", base_filename, i);
- chan->buf[i] = relay_open_buf(chan, tmpname, parent,
- &is_global);
+ chan->buf[i] = relay_open_buf(chan, i);
if (!chan->buf[i])
goto free_bufs;
-
- chan->buf[i]->cpu = i;
}
+ list_add(&chan->list, &relay_channels);
+ mutex_unlock(&relay_channels_mutex);

- kfree(tmpname);
return chan;

free_bufs:
- for (i = 0; i < NR_CPUS; i++) {
+ for_each_online_cpu(i) {
if (!chan->buf[i])
break;
relay_close_buf(chan->buf[i]);
- if (is_global)
- break;
}
- kfree(tmpname);

-free_chan:
kref_put(&chan->kref, relay_destroy_channel);
+ mutex_unlock(&relay_channels_mutex);
return NULL;
}
EXPORT_SYMBOL_GPL(relay_open);
@@ -617,24 +677,26 @@ EXPORT_SYMBOL_GPL(relay_subbufs_consumed
void relay_close(struct rchan *chan)
{
unsigned int i;
- struct rchan_buf *prev = NULL;

if (!chan)
return;

- for (i = 0; i < NR_CPUS; i++) {
- if (!chan->buf[i] || chan->buf[i] == prev)
- break;
- relay_close_buf(chan->buf[i]);
- prev = chan->buf[i];
- }
+ mutex_lock(&relay_channels_mutex);
+ if (chan->is_global && chan->buf[0])
+ relay_close_buf(chan->buf[0]);
+ else
+ for_each_possible_cpu(i)
+ if (chan->buf[i])
+ relay_close_buf(chan->buf[i]);

if (chan->last_toobig)
printk(KERN_WARNING "relay: one or more items not logged "
"[item size (%Zd) > sub-buffer size (%Zd)]\n",
chan->last_toobig, chan->subbuf_size);

+ list_del(&chan->list);
kref_put(&chan->kref, relay_destroy_channel);
+ mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_close);

@@ -647,17 +709,20 @@ EXPORT_SYMBOL_GPL(relay_close);
void relay_flush(struct rchan *chan)
{
unsigned int i;
- struct rchan_buf *prev = NULL;

if (!chan)
return;

- for (i = 0; i < NR_CPUS; i++) {
- if (!chan->buf[i] || chan->buf[i] == prev)
- break;
- relay_switch_subbuf(chan->buf[i], 0);
- prev = chan->buf[i];
+ if (chan->is_global && chan->buf[0]) {
+ relay_switch_subbuf(chan->buf[0], 0);
+ return;
}
+
+ mutex_lock(&relay_channels_mutex);
+ for_each_possible_cpu(i)
+ if (chan->buf[i])
+ relay_switch_subbuf(chan->buf[i], 0);
+ mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_flush);

@@ -1021,3 +1086,12 @@ struct file_operations relay_file_operat
.sendfile = relay_file_sendfile,
};
EXPORT_SYMBOL_GPL(relay_file_operations);
+
+static __init int relay_init(void)
+{
+
+ hotcpu_notifier(relay_hotcpu_callback, 0);
+ return 0;
+}
+
+module_init(relay_init);



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