Re: [PATCH] nfsd: Reduce contention for the nfsd_file nf_rwsem

[Chuck Lever III <chuck.lever@oracle.com>]

> On Jun 17, 2021, at 7:26 PM, trondmy@kernel.org wrote:
> 
> From: Trond Myklebust <trond.myklebust@hammerspace.com>
> 
> When flushing out the unstable file writes as part of a COMMIT call, try
> to perform most of of the data writes and waits outside the semaphore.
> 
> This means that if the client is sending the COMMIT as part of a memory
> reclaim operation, then it can continue performing I/O, with contention
> for the lock occurring only once the data sync is finished.
> 
> Fixes: 5011af4c698a ("nfsd: Fix stable writes")
> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

I can see write throughput improvement now.

Tested-by: Chuck Lever <chuck.lever@oracle.com>


This is NFSv3 against an NVMe-backed XFS export, wsize=8192:

v5.13-rc6:

	Command line used: /home/cel/bin/iozone -M -+u -i0 -i1 -s1g -r256k -t12 -I
	Output is in kBytes/sec
	Time Resolution = 0.000001 seconds.
	Processor cache size set to 1024 kBytes.
	Processor cache line size set to 32 bytes.
	File stride size set to 17 * record size.
	Throughput test with 12 processes
	Each process writes a 1048576 kByte file in 256 kByte records

	Children see throughput for 12 initial writers 	=  416004.59 kB/sec
	Parent sees throughput for 12 initial writers 	=  415691.65 kB/sec
	Min throughput per process 			=   34630.36 kB/sec 
	Max throughput per process 			=   34703.62 kB/sec
	Avg throughput per process 			=   34667.05 kB/sec
	Min xfer 					= 1046528.00 kB
	CPU Utilization: Wall time   30.239    CPU time    5.854    CPU utilization  19.36 %


	Children see throughput for 12 rewriters 	=  516605.59 kB/sec
	Parent sees throughput for 12 rewriters 	=  516530.05 kB/sec
	Min throughput per process 			=   43007.56 kB/sec 
	Max throughput per process 			=   43074.50 kB/sec
	Avg throughput per process 			=   43050.47 kB/sec
	Min xfer 					= 1047040.00 kB
	CPU utilization: Wall time   24.347    CPU time    5.882    CPU utilization  24.16 %

v5.13-rc6 + Trond's patch:

	Command line used: /home/cel/bin/iozone -M -+u -i0 -i1 -s1g -r256k -t12 -I
	Output is in kBytes/sec
	Time Resolution = 0.000001 seconds.
	Processor cache size set to 1024 kBytes.
	Processor cache line size set to 32 bytes.
	File stride size set to 17 * record size.
	Throughput test with 12 processes
	Each process writes a 1048576 kByte file in 256 kByte records

	Children see throughput for 12 initial writers 	=  434971.09 kB/sec
	Parent sees throughput for 12 initial writers 	=  434649.13 kB/sec
	Min throughput per process 			=   36209.41 kB/sec 
	Max throughput per process 			=   36287.55 kB/sec
	Avg throughput per process 			=   36247.59 kB/sec
	Min xfer 					= 1046528.00 kB
	CPU Utilization: Wall time   28.920    CPU time    5.705    CPU utilization  19.73 %


	Children see throughput for 12 rewriters 	=  544700.37 kB/sec
	Parent sees throughput for 12 rewriters 	=  544623.91 kB/sec
	Min throughput per process 			=   45320.82 kB/sec 
	Max throughput per process 			=   45456.07 kB/sec
	Avg throughput per process 			=   45391.70 kB/sec
	Min xfer 					= 1045504.00 kB
	CPU utilization: Wall time   23.071    CPU time    5.708    CPU utilization  24.74 %


> ---
> fs/nfsd/vfs.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 15adf1f6ab21..46485c04740d 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1123,6 +1123,19 @@ nfsd_write(struct svc_rqst *rqstp, struct svc_fh *fhp, loff_t offset,
> }
> 
> #ifdef CONFIG_NFSD_V3
> +static int
> +nfsd_filemap_write_and_wait_range(struct nfsd_file *nf, loff_t offset,
> +				  loff_t end)
> +{
> +	struct address_space *mapping = nf->nf_file->f_mapping;
> +	int ret = filemap_fdatawrite_range(mapping, offset, end);
> +
> +	if (ret)
> +		return ret;
> +	filemap_fdatawait_range_keep_errors(mapping, offset, end);
> +	return 0;
> +}
> +
> /*
>  * Commit all pending writes to stable storage.
>  *
> @@ -1153,10 +1166,11 @@ nfsd_commit(struct svc_rqst *rqstp, struct svc_fh *fhp,
> 	if (err)
> 		goto out;
> 	if (EX_ISSYNC(fhp->fh_export)) {
> -		int err2;
> +		int err2 = nfsd_filemap_write_and_wait_range(nf, offset, end);
> 
> 		down_write(&nf->nf_rwsem);
> -		err2 = vfs_fsync_range(nf->nf_file, offset, end, 0);
> +		if (!err2)
> +			err2 = vfs_fsync_range(nf->nf_file, offset, end, 0);
> 		switch (err2) {
> 		case 0:
> 			nfsd_copy_boot_verifier(verf, net_generic(nf->nf_net,
> -- 
> 2.31.1
> 

--
Chuck Lever