On Feb 8, 2013, at 3:55 PM, "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> On Fri, Feb 08, 2013 at 08:27:06AM -0500, Jeff Layton wrote:
>> On Thu, 7 Feb 2013 13:03:16 -0500
>> Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>>
>>> On Thu, 7 Feb 2013 10:51:02 -0500
>>> Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:
>>>
>>>>
>>>> On Feb 7, 2013, at 9:51 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>>>>
>>>>> Now that we're allowing more DRC entries, it becomes a lot easier to hit
>>>>> problems with XID collisions. In order to mitigate those, calculate the
>>>>> crc32 of up to the first 256 bytes of each request coming in and store
>>>>> that in the cache entry, along with the total length of the request.
>>>>
>>>> I'm happy to see a checksummed DRC finally become reality for the Linux NFS server.
>>>>
>>>> Have you measured the CPU utilization impact and CPU cache footprint of performing a CRC computation for every incoming RPC? I'm wondering if a simpler checksum might be just as useful but less costly to compute.
>>>>
>>>
>>> No, I haven't, at least not in any sort of rigorous way. It's pretty
>>> negligible on "normal" PC hardware, but I think most intel and amd cpus
>>> have instructions for handling crc32. I'm ok with a different checksum,
>>> we don't need anything cryptographically secure here. I simply chose
>>> crc32 since it has an easily available API, and I figured it would be
>>> fairly lightweight.
>>>
>>
>> After an abortive attempt to measure this with ftrace, I ended up
>> hacking together a patch to just measure the latency of the
>> nfsd_cache_csum/_crc functions to get some rough numbers. On my x86_64
>> KVM guest, the avg time to calculate the crc32 is ~1750ns. Using IP
>> checksums cuts that roughly in half to ~800ns. I'm not sure how best to
>> measure the cache footprint however.
>>
>> Neither seems terribly significant, especially given the other
>> inefficiencies in this code. OTOH, I guess those latencies can add up,
>> and I don't see any need to use crc32 over the net/checksum.h routines.
>> We probably ought to go with my RFC patch from yesterday.
>
> OK, I hadn't committed the original yet, so I've just rolled them
> together and added a little of the above to the changelog. Look OK?
> Chuck, should I add a Reviewed-by: ?
Not sure my participation counts as review. How about:
Stones-thrown-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> --b.
>
> commit a937bd422ccc4306cdc81b5aa60b12a7212b70d3
> Author: Jeff Layton <jlayton@xxxxxxxxxx>
> Date: Mon Feb 4 11:57:27 2013 -0500
>
> nfsd: keep a checksum of the first 256 bytes of request
>
> Now that we're allowing more DRC entries, it becomes a lot easier to hit
> problems with XID collisions. In order to mitigate those, calculate a
> checksum of up to the first 256 bytes of each request coming in and store
> that in the cache entry, along with the total length of the request.
>
> This initially used crc32, but Chuck Lever and Jim Rees pointed out that
> crc32 is probably more heavyweight than we really need for generating
> these checksums, and recommended looking at using the same routines that
> are used to generate checksums for IP packets.
>
> On an x86_64 KVM guest measurements with ftrace showed ~800ns to use
> csum_partial vs ~1750ns for crc32. The difference probably isn't
> terribly significant, but for now we may as well use csum_partial.
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
>
> diff --git a/fs/nfsd/cache.h b/fs/nfsd/cache.h
> index 9c7232b..87fd141 100644
> --- a/fs/nfsd/cache.h
> +++ b/fs/nfsd/cache.h
> @@ -29,6 +29,8 @@ struct svc_cacherep {
> u32 c_prot;
> u32 c_proc;
> u32 c_vers;
> + unsigned int c_len;
> + __wsum c_csum;
> unsigned long c_timestamp;
> union {
> struct kvec u_vec;
> @@ -73,6 +75,9 @@ enum {
> /* Cache entries expire after this time period */
> #define RC_EXPIRE (120 * HZ)
>
> +/* Checksum this amount of the request */
> +#define RC_CSUMLEN (256U)
> +
> int nfsd_reply_cache_init(void);
> void nfsd_reply_cache_shutdown(void);
> int nfsd_cache_lookup(struct svc_rqst *);
> diff --git a/fs/nfsd/nfscache.c b/fs/nfsd/nfscache.c
> index f754469..40db57e 100644
> --- a/fs/nfsd/nfscache.c
> +++ b/fs/nfsd/nfscache.c
> @@ -11,6 +11,7 @@
> #include <linux/slab.h>
> #include <linux/sunrpc/addr.h>
> #include <linux/highmem.h>
> +#include <net/checksum.h>
>
> #include "nfsd.h"
> #include "cache.h"
> @@ -130,6 +131,7 @@ int nfsd_reply_cache_init(void)
> INIT_LIST_HEAD(&lru_head);
> max_drc_entries = nfsd_cache_size_limit();
> num_drc_entries = 0;
> +
> return 0;
> out_nomem:
> printk(KERN_ERR "nfsd: failed to allocate reply cache\n");
> @@ -238,12 +240,45 @@ nfsd_reply_cache_shrink(struct shrinker *shrink, struct shrink_control *sc)
> }
>
> /*
> + * Walk an xdr_buf and get a CRC for at most the first RC_CSUMLEN bytes
> + */
> +static __wsum
> +nfsd_cache_csum(struct svc_rqst *rqstp)
> +{
> + int idx;
> + unsigned int base;
> + __wsum csum;
> + struct xdr_buf *buf = &rqstp->rq_arg;
> + const unsigned char *p = buf->head[0].iov_base;
> + size_t csum_len = min_t(size_t, buf->head[0].iov_len + buf->page_len,
> + RC_CSUMLEN);
> + size_t len = min(buf->head[0].iov_len, csum_len);
> +
> + /* rq_arg.head first */
> + csum = csum_partial(p, len, 0);
> + csum_len -= len;
> +
> + /* Continue into page array */
> + idx = buf->page_base / PAGE_SIZE;
> + base = buf->page_base & ~PAGE_MASK;
> + while (csum_len) {
> + p = page_address(buf->pages[idx]) + base;
> + len = min(PAGE_SIZE - base, csum_len);
> + csum = csum_partial(p, len, csum);
> + csum_len -= len;
> + base = 0;
> + ++idx;
> + }
> + return csum;
> +}
> +
> +/*
> * Search the request hash for an entry that matches the given rqstp.
> * Must be called with cache_lock held. Returns the found entry or
> * NULL on failure.
> */
> static struct svc_cacherep *
> -nfsd_cache_search(struct svc_rqst *rqstp)
> +nfsd_cache_search(struct svc_rqst *rqstp, __wsum csum)
> {
> struct svc_cacherep *rp;
> struct hlist_node *hn;
> @@ -257,6 +292,7 @@ nfsd_cache_search(struct svc_rqst *rqstp)
> hlist_for_each_entry(rp, hn, rh, c_hash) {
> if (xid == rp->c_xid && proc == rp->c_proc &&
> proto == rp->c_prot && vers == rp->c_vers &&
> + rqstp->rq_arg.len == rp->c_len && csum == rp->c_csum &&
> rpc_cmp_addr(svc_addr(rqstp), (struct sockaddr *)&rp->c_addr) &&
> rpc_get_port(svc_addr(rqstp)) == rpc_get_port((struct sockaddr *)&rp->c_addr))
> return rp;
> @@ -277,6 +313,7 @@ nfsd_cache_lookup(struct svc_rqst *rqstp)
> u32 proto = rqstp->rq_prot,
> vers = rqstp->rq_vers,
> proc = rqstp->rq_proc;
> + __wsum csum;
> unsigned long age;
> int type = rqstp->rq_cachetype;
> int rtn;
> @@ -287,10 +324,12 @@ nfsd_cache_lookup(struct svc_rqst *rqstp)
> return RC_DOIT;
> }
>
> + csum = nfsd_cache_csum(rqstp);
> +
> spin_lock(&cache_lock);
> rtn = RC_DOIT;
>
> - rp = nfsd_cache_search(rqstp);
> + rp = nfsd_cache_search(rqstp, csum);
> if (rp)
> goto found_entry;
>
> @@ -318,7 +357,7 @@ nfsd_cache_lookup(struct svc_rqst *rqstp)
> * Must search again just in case someone inserted one
> * after we dropped the lock above.
> */
> - found = nfsd_cache_search(rqstp);
> + found = nfsd_cache_search(rqstp, csum);
> if (found) {
> nfsd_reply_cache_free_locked(rp);
> rp = found;
> @@ -344,6 +383,8 @@ setup_entry:
> rpc_set_port((struct sockaddr *)&rp->c_addr, rpc_get_port(svc_addr(rqstp)));
> rp->c_prot = proto;
> rp->c_vers = vers;
> + rp->c_len = rqstp->rq_arg.len;
> + rp->c_csum = csum;
>
> hash_refile(rp);
> lru_put_end(rp);
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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