linux-kernel - Re: [RFC PATCH-tip 6/6] xfs: Enable reader optimistic spinning for DAX inodes

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Date:	Wed, 15 Jun 2016 14:55:11 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <linux-alpha@...r.kernel.org>,
	<linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, <xfs@....sgi.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip 6/6] xfs: Enable reader optimistic spinning for
 DAX inodes

On 06/14/2016 07:06 PM, Dave Chinner wrote:
> On Tue, Jun 14, 2016 at 02:12:39PM -0400, Waiman Long wrote:
>> This patch enables reader optimistic spinning for inodes that are
>> under a DAX-based mount point.
>>
>> On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel,
>> the fio test with multithreaded randrw and randwrite tests on the
>> same file on a XFS partition on top of a NVDIMM with DAX were run,
>> the aggregated bandwidths before and after the patch were as follows:
>>
>>    Test      BW before patch     BW after patch  % change
>>    ----      ---------------     --------------  --------
>>    randrw        1352 MB/s          2164 MB/s      +60%
>>    randwrite     1710 MB/s          2550 MB/s      +49%
>>
>> Signed-off-by: Waiman Long<Waiman.Long@....com>
>> ---
>>   fs/xfs/xfs_icache.c |    9 +++++++++
>>   1 files changed, 9 insertions(+), 0 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
>> index 99ee6ee..09f284f 100644
>> --- a/fs/xfs/xfs_icache.c
>> +++ b/fs/xfs/xfs_icache.c
>> @@ -71,6 +71,15 @@ xfs_inode_alloc(
>>
>>   	mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
>>
>> +	/*
>> +	 * Enable reader spinning for DAX nount point
>> +	 */
>> +	if (mp->m_flags&  XFS_MOUNT_DAX) {
>> +		rwsem_set_rspin_threshold(&ip->i_iolock.mr_lock);
>> +		rwsem_set_rspin_threshold(&ip->i_mmaplock.mr_lock);
>> +		rwsem_set_rspin_threshold(&ip->i_lock.mr_lock);
>> +	}
> That's wrong. DAX is a per-inode flag, not a mount wide flag. This
> needs to be done once the inode has been fully initialised and
> IS_DAX(inode) can be run.
>
> Also, the benchmark doesn't show that all these locks are being
> tested by this benchmark. e.g. the i_mmaplock isn't involved in
> the benchmark's IO paths at all. It's only taken in page faults and
> truncate paths....
>
> I'd also like to see how much of the gain comes from the iolock vs
> the ilock, as the ilock is nested inside the iolock and so
> contention is much rarer....

This patch has now been superseded by a second one where changes to the 
xfs code is no longer needed. The new patch will enable reader spinning 
for all rwsem and dynamically disable it depending on past history.

> As it is, I'm *extremely* paranoid when it comes to changes to core
> locking like this. Performance is secondary to correctness, and we
> need much more than just a few benchmarks to verify there aren't
> locking bugs being introduced....

The core rwsem locking logic hasn't been changed. There are some minor 
changes, however, on what RWSEM_WAITING_BIAS value to use that requires 
more eyeballs to make sure that it hasn't introduced any new bug.

Cheers,
Longman