Return-Path: linux-nfs-owner@vger.kernel.org Received: from fieldses.org ([174.143.236.118]:50842 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1947068Ab3BHVCp (ORCPT ); Fri, 8 Feb 2013 16:02:45 -0500 Date: Fri, 8 Feb 2013 16:02:43 -0500 From: "J. Bruce Fields" To: Chuck Lever Cc: Jeff Layton , linux-nfs@vger.kernel.org, rees@umich.edu Subject: Re: [PATCH v3 2/2] nfsd: keep a checksum of the first 256 bytes of request Message-ID: <20130208210242.GE21040@fieldses.org> References: <1360248701-23963-1-git-send-email-jlayton@redhat.com> <1360248701-23963-3-git-send-email-jlayton@redhat.com> <20130207130316.0cf29993@corrin.poochiereds.net> <20130208082706.614035e2@tlielax.poochiereds.net> <20130208205555.GC21040@fieldses.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Sender: linux-nfs-owner@vger.kernel.org List-ID: On Fri, Feb 08, 2013 at 03:59:53PM -0500, Chuck Lever wrote: > > On Feb 8, 2013, at 3:55 PM, "J. Bruce Fields" 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 wrote: > >> > >>> On Thu, 7 Feb 2013 10:51:02 -0500 > >>> Chuck Lever wrote: > >>> > >>>> > >>>> On Feb 7, 2013, at 9:51 AM, Jeff Layton 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 As you wish.--b. > > > --b. > > > > commit a937bd422ccc4306cdc81b5aa60b12a7212b70d3 > > Author: Jeff Layton > > 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 > > Signed-off-by: J. Bruce Fields > > > > 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 > > #include > > #include > > +#include > > > > #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 > > > >