linux-kernel - [PATCH 6/9] SUNRPC: Set rq_page

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Date:   Thu, 25 Mar 2021 11:42:25 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-NFS <linux-nfs@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 6/9] SUNRPC: Set rq_page_end differently

From: Chuck Lever <chuck.lever@...cle.com>

Patch series "SUNRPC consumer for the bulk page allocator"

This patch set and the measurements below are based on yesterday's
bulk allocator series:

git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-bulk-rebase-v5r9

The patches change SUNRPC to invoke the array-based bulk allocator
instead of alloc_page().

The micro-benchmark results are promising. I ran a mixture of 256KB
reads and writes over NFSv3. The server's kernel is built with KASAN
enabled, so the comparison is exaggerated but I believe it is still
valid.

I instrumented svc_recv() to measure the latency of each call to
svc_alloc_arg() and report it via a trace point. The following
results are averages across the trace events.

Single page: 25.007 us per call over 532,571 calls
Bulk list:    6.258 us per call over 517,034 calls
Bulk array:   4.590 us per call over 517,442 calls

This patch (of 2)

Refactor:

I'm about to use the loop variable @i for something else.

As far as the "i++" is concerned, that is a post-increment. The
value of @i is not used subsequently, so the increment operator
is unnecessary and can be removed.

Also note that nfsd_read_actor() was renamed nfsd_splice_actor()
by commit cf8208d0eabd ("sendfile: convert nfsd to
splice_direct_to_actor()").

Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 net/sunrpc/svc_xprt.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 3cdd71a8df1e..609bda97d4ae 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -642,7 +642,7 @@ static void svc_check_conn_limits(struct svc_serv *serv)
 static int svc_alloc_arg(struct svc_rqst *rqstp)
 {
 	struct svc_serv *serv = rqstp->rq_server;
-	struct xdr_buf *arg;
+	struct xdr_buf *arg = &rqstp->rq_arg;
 	int pages;
 	int i;
 
@@ -667,11 +667,10 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
 			}
 			rqstp->rq_pages[i] = p;
 		}
-	rqstp->rq_page_end = &rqstp->rq_pages[i];
-	rqstp->rq_pages[i++] = NULL; /* this might be seen in nfs_read_actor */
+	rqstp->rq_page_end = &rqstp->rq_pages[pages];
+	rqstp->rq_pages[pages] = NULL; /* this might be seen in nfsd_splice_actor() */
 
 	/* Make arg->head point to first page and arg->pages point to rest */
-	arg = &rqstp->rq_arg;
 	arg->head[0].iov_base = page_address(rqstp->rq_pages[0]);
 	arg->head[0].iov_len = PAGE_SIZE;
 	arg->pages = rqstp->rq_pages + 1;
-- 
2.26.2