On Apr. 21, 2010, 18:22 +0300, "William A. (Andy) Adamson" <androsadamson@xxxxxxxxx> wrote:
> On Wed, Apr 21, 2010 at 1:59 AM, Benny Halevy <bhalevy@xxxxxxxxxxx> wrote:
>> On Apr. 16, 2010, 19:04 +0300, "William A. (Andy) Adamson" <androsadamson@xxxxxxxxx> wrote:
>>> On Fri, Apr 16, 2010 at 11:52 AM, <andros@xxxxxxxxxx> wrote:
>>>> From: Andy Adamson <andros@xxxxxxxxxx>
>>>>
>>>> A shared RCU device ID cache servicing multiple mounts of a single layout type
>>>> per meta data server (struct nfs_client).
>>>>
>>>> Device IDs of type deviceid4 are required by all layout types, long lived and
>>>> read at each I/O. They are added to the deviceid cache at first reference by
>>>> a layout via GETDEVICEINFO and (currently) are only removed at umount.
>>>>
>>>> Reference count the device ID cache for each mounted file system
>>>> in the initialize_mountpoint layoutdriver_io_operation.
>>>>
>>>> Dereference the device id cache on file system in the uninitialize_mountpoint
>>>> layoutdriver_io_operation called at umount
>>>>
>>>> Signed-off-by: Andy Adamson <andros@xxxxxxxxxx>
>>>> ---
>>>> fs/nfs/pnfs.c | 119 +++++++++++++++++++++++++++++++++++++++++++++
>>>> include/linux/nfs4_pnfs.h | 27 ++++++++++
>>>> include/linux/nfs_fs_sb.h | 1 +
>>>> 3 files changed, 147 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
>>>> index 91572aa..8492aef 100644
>>>> --- a/fs/nfs/pnfs.c
>>>> +++ b/fs/nfs/pnfs.c
>>>> @@ -45,6 +45,7 @@
>>>> #include <linux/nfs4.h>
>>>> #include <linux/pnfs_xdr.h>
>>>> #include <linux/nfs4_pnfs.h>
>>>> +#include <linux/rculist.h>
>>>>
>>>> #include "internal.h"
>>>> #include "nfs4_fs.h"
>>>> @@ -2296,3 +2297,121 @@ struct pnfs_client_operations pnfs_ops = {
>>>>
>>>> EXPORT_SYMBOL(pnfs_unregister_layoutdriver);
>>>> EXPORT_SYMBOL(pnfs_register_layoutdriver);
>>>> +
>>>> +
>>>> +/* Device ID cache. Supports one layout type per struct nfs_client */
>>>> +int
>>>> +nfs4_alloc_init_deviceid_cache(struct nfs_client *clp,
>>>> + void (*free_callback)(struct rcu_head *))
>>>> +{
>>>> + struct nfs4_deviceid_cache *c;
>>>> +
>>>> + c = kzalloc(sizeof(struct nfs4_deviceid_cache), GFP_KERNEL);
>>>> + if (!c)
>>>> + return -ENOMEM;
>>>> + spin_lock(&clp->cl_lock);
>>>> + if (clp->cl_devid_cache != NULL) {
>>>> + kref_get(&clp->cl_devid_cache->dc_kref);
>>>> + spin_unlock(&clp->cl_lock);
>>>> + dprintk("%s [kref [%d]]\n", __func__,
>>>> + atomic_read(&clp->cl_devid_cache->dc_kref.refcount));
>>>> + kfree(c);
>>>> + } else {
>>>> + spin_lock_init(&c->dc_lock);
>>>> + INIT_LIST_HEAD(&c->dc_deviceids);
>>>> + kref_init(&c->dc_kref);
>>>> + c->dc_free_callback = free_callback;
>>>> + c->dc_clp = clp;
>>>> + clp->cl_devid_cache = c;
>>>> + spin_unlock(&clp->cl_lock);
>>>> + dprintk("%s [new]\n", __func__);
>>>> + }
>>>> + return 0;
>>>> +}
>>>> +EXPORT_SYMBOL(nfs4_alloc_init_deviceid_cache);
>>>> +
>>>> +void
>>>> +nfs4_init_deviceid_node(struct nfs4_deviceid *d)
>>>> +{
>>>> + INIT_LIST_HEAD(&d->de_node);
>>>> + INIT_RCU_HEAD(&d->de_rcu);
>>>> +}
>>>> +EXPORT_SYMBOL(nfs4_init_deviceid_node);
>>>> +
>>>> +struct nfs4_deviceid *
>>>> +nfs4_find_deviceid(struct nfs4_deviceid_cache *c, struct pnfs_deviceid *id)
>>>> +{
>>>> + struct nfs4_deviceid *d;
>>>> +
>>>> + rcu_read_lock();
>>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>>> + if (!memcmp(&d->de_id, id, NFS4_PNFS_DEVICEID4_SIZE)) {
>>>> + rcu_read_unlock();
>>>> + return d;
>>>> + }
>>>> + }
>>>> + rcu_read_unlock();
>>
>> I hope this is worth the added complexity...
>>
>> Out of curiosity, do you have a benchmark comparing the cost
>> of rcu_read_lock (and unlock) vs. spin_lock(&c->dc_lock)?
>
> The deviceid cache is read at each I/O. If we use a spin_lock to
Yeah, I see where this goes...
In the objects layout driver we get a reference on the device structure
at alloc_lseg time and keep a pointer to it throughout the lseg's life time.
This saves the deviceid lookup on every I/O.
Benny
> protect the deviceid cache, this would mean that all I/0 is serialized
> behind the spin_lock even though the deviceid cache is changed
> infrequently. The RCU allows readers to "run almost naked" and does
> not serialize I/O behind reading the deviceid cache.
>
> So I think it is worth it. I have not benchmarked the difference.
>
>
>>
>>>> + return NULL;
>>>> +}
>>>> +EXPORT_SYMBOL(nfs4_find_deviceid);
>>>> +
>>>> +/*
>>>> + * Add or kref_get a deviceid.
>>>> + * GETDEVICEINFOs for same deviceid can race. If deviceid is found, discard new
>>>> + */
>>>> +void
>>>> +nfs4_add_deviceid(struct nfs4_deviceid_cache *c, struct nfs4_deviceid *new)
>>>> +{
>>>> + struct nfs4_deviceid *d;
>>>> +
>>>> + spin_lock(&c->dc_lock);
>>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>>> + if (!memcmp(&d->de_id, &new->de_id, NFS4_PNFS_DEVICEID4_SIZE)) {
>>>> + spin_unlock(&c->dc_lock);
>>>> + dprintk("%s [discard]\n", __func__);
>>>> + c->dc_free_callback(&new->de_rcu);
>>>> + }
>>>> + }
>>>> + list_add_rcu(&new->de_node, &c->dc_deviceids);
>>>> + spin_unlock(&c->dc_lock);
>>>> + dprintk("%s [new]\n", __func__);
>>>> +}
>>>> +EXPORT_SYMBOL(nfs4_add_deviceid);
>>>> +
>>>> +static int
>>>> +nfs4_remove_deviceid(struct nfs4_deviceid_cache *c)
>>>> +{
>>>> + struct nfs4_deviceid *d;
>>>> +
>>>> + spin_lock(&c->dc_lock);
>>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>>> + list_del_rcu(&d->de_node);
>>>> + spin_unlock(&c->dc_lock);
>>>> + synchronize_rcu();
>>>> + c->dc_free_callback(&d->de_rcu);
>>>> + return 1;
>>>> + }
>>>> + spin_unlock(&c->dc_lock);
>>>> + return 0;
>>>> +}
>>>> +
>>>> +static void
>>>> +nfs4_free_deviceid_cache(struct kref *kref)
>>>> +{
>>>> + struct nfs4_deviceid_cache *cache =
>>>> + container_of(kref, struct nfs4_deviceid_cache, dc_kref);
>>>> + int more = 1;
>>>> +
>>>> + while (more)
>>>> + more = nfs4_remove_deviceid(cache);
>>>> + cache->dc_clp->cl_devid_cache = NULL;
>>>
>>> Need to take the cl_lock around this assignment
>>>
>>> spin_lock(&cache->dc_clp->cl_lock);
>>> cache->dc_clp->cl_devid_cache = NULL
>>> spin_unlock(&cache->dc_clp->cl_lock);
>>>
>>>
>>
>> That must happen atomically before kref_put.
>> It's illegal to have cl_devid_cache be referenced by cache->dc_clp
>> without a reference count backing it up.
>> Otherwise, if accessed concurrently to this piece of code
>> someone might call kref_get while the refcount is zero.
>>
>> Normally, you'd first clear the referencing pointer to prevent any
>> new reference to it and only then kref_put it, e.g.:
>>
>> spin_lock(&cache->dc_clp->cl_lock);
>> tmp = cache->dc_clp->cl_devid_cache;
>> cache->dc_clp->cl_devid_cache = NULL
>> spin_unlock(&cache->dc_clp->cl_lock);
>> kref_put(&tmp->dc_kref, nfs4_free_deviceid_cache);
>
> Good point. Thanks for the review. I'll rethink and resend
>
> -->Andy
>
>>
>> Benny
>>
>>>> + kfree(cache);
>>>> +}
>>>> +
>>>> +void
>>>> +nfs4_put_deviceid_cache(struct nfs4_deviceid_cache *c)
>>>> +{
>>>> + dprintk("%s [%d]\n", __func__, atomic_read(&c->dc_kref.refcount));
>>>> + kref_put(&c->dc_kref, nfs4_free_deviceid_cache);
>>>> +}
>>>> +EXPORT_SYMBOL(nfs4_put_deviceid_cache);
>>>> diff --git a/include/linux/nfs4_pnfs.h b/include/linux/nfs4_pnfs.h
>>>> index 1d521f4..2a88a06 100644
>>>> --- a/include/linux/nfs4_pnfs.h
>>>> +++ b/include/linux/nfs4_pnfs.h
>>>> @@ -281,6 +281,33 @@ struct pnfs_devicelist {
>>>> struct pnfs_deviceid dev_id[NFS4_PNFS_GETDEVLIST_MAXNUM];
>>>> };
>>>>
>>>> +/*
>>>> + * Device ID RCU cache. A device ID is unique per client ID and layout type.
>>>> + */
>>>> +struct nfs4_deviceid_cache {
>>>> + spinlock_t dc_lock;
>>>> + struct list_head dc_deviceids;
>>>> + struct kref dc_kref;
>>>> + void (*dc_free_callback)(struct rcu_head *);
>>>> + struct nfs_client *dc_clp;
>>>> +};
>>>> +
>>>> +/* Device ID cache node */
>>>> +struct nfs4_deviceid {
>>>> + struct list_head de_node;
>>>> + struct rcu_head de_rcu;
>>>> + struct pnfs_deviceid de_id;
>>>> +};
>>>> +
>>>> +extern int nfs4_alloc_init_deviceid_cache(struct nfs_client *,
>>>> + void (*free_callback)(struct rcu_head *));
>>>> +extern void nfs4_put_deviceid_cache(struct nfs4_deviceid_cache *);
>>>> +extern void nfs4_init_deviceid_node(struct nfs4_deviceid *);
>>>> +extern struct nfs4_deviceid *nfs4_find_deviceid(struct nfs4_deviceid_cache *,
>>>> + struct pnfs_deviceid *);
>>>> +extern void nfs4_add_deviceid(struct nfs4_deviceid_cache *,
>>>> + struct nfs4_deviceid *);
>>>> +
>>>> /* pNFS client callback functions.
>>>> * These operations allow the layout driver to access pNFS client
>>>> * specific information or call pNFS client->server operations.
>>>> diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
>>>> index 8522461..ef2e18e 100644
>>>> --- a/include/linux/nfs_fs_sb.h
>>>> +++ b/include/linux/nfs_fs_sb.h
>>>> @@ -87,6 +87,7 @@ struct nfs_client {
>>>> u32 cl_exchange_flags;
>>>> struct nfs4_session *cl_session; /* sharred session */
>>>> struct list_head cl_lo_inodes; /* Inodes having layouts */
>>>> + struct nfs4_deviceid_cache *cl_devid_cache; /* pNFS deviceid cache */
>>>> #endif /* CONFIG_NFS_V4_1 */
>>>>
>>>> #ifdef CONFIG_NFS_FSCACHE
>>>> --
>>>> 1.6.6
>>>>
>>>> --
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>>>>
>>> _______________________________________________
>>> pNFS mailing list
>>> pNFS@xxxxxxxxxxxxx
>>> http://linux-nfs.org/cgi-bin/mailman/listinfo/pnfs
>>
--
Benny Halevy
Software Architect
Panasas, Inc.
bhalevy@xxxxxxxxxxx
Tel/Fax: +972-3-647-8340
Mobile: +972-54-802-8340
Panasas: The Leader in Parallel Storage
www.panasas.com
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