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From: Bob Peterson <rpeterso@redhat.com>
To: Steven Whitehouse <swhiteho@redhat.com>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <djwong@kernel.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-kernel@vger.kernel.org, cluster-devel@redhat.com,
	linux-fsdevel@vger.kernel.org, ocfs2-devel@oss.oracle.com
Subject: Re: [Ocfs2-devel] [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion
Date: Fri, 20 Aug 2021 08:11:34 -0500	[thread overview]
Message-ID: <cf284633-a9db-9f88-6b60-4377bc33e473@redhat.com> (raw)
In-Reply-To: <5e8a20a8d45043e88013c6004636eae5dadc9be3.camel@redhat.com>

On 8/20/21 4:35 AM, Steven Whitehouse wrote:
> Hi,
> 
> On Thu, 2021-08-19 at 21:40 +0200, Andreas Gruenbacher wrote:
>> From: Bob Peterson <rpeterso@redhat.com>
>>
>> This patch introduces a new HIF_MAY_DEMOTE flag and infrastructure
>> that
>> will allow glocks to be demoted automatically on locking conflicts.
>> When a locking request comes in that isn't compatible with the
>> locking
>> state of a holder and that holder has the HIF_MAY_DEMOTE flag set,
>> the
>> holder will be demoted automatically before the incoming locking
>> request
>> is granted.
>>
> I'm not sure I understand what is going on here. When there are locking
> conflicts we generate call backs and those result in glock demotion.
> There is no need for a flag to indicate that I think, since it is the
> default behaviour anyway. Or perhaps the explanation is just a bit
> confusing...

I agree that the whole concept and explanation are confusing. Andreas 
and I went through several heated arguments about the symantics, 
comments, patch descriptions, etc. We played around with many different 
flag name ideas, etc. We did not agree on the best way to describe the 
whole concept. He didn't like my explanation and I didn't like his. So 
yes, it is confusing.

My preferred terminology was "DOD" or "Dequeue On Demand" which makes 
the concept more understandable to me. So basically a process can say
"I need to hold this glock, but for an unknown and possibly lengthy 
period of time, but please feel free to dequeue it if it's in your way."
And bear in mind that several processes may do the same, simultaneously.

You can almost think of this as a performance enhancement. This concept 
allows a process to hold a glock for much longer periods of time, at a 
lower priority, for example, when gfs2_file_read_iter needs to hold the 
glock for very long-running iterative reads.

The process requesting a holder with "Demote On Demand" must then 
determine if its holder has been stolen away (dequeued on demand) after 
its lengthy operation, and therefore needs to pick up the pieces of 
where it left off in its process.

Meanwhile, another process may need to hold the glock. If its requested 
mode is compatible, say SH and SH, the lock is simply granted with no 
further delay. If the mode is incompatible, regardless of whether it's 
on the local node or a different node in the cluster, these 
longer-term/lower-priority holders may be dequeued or prempted by 
another request to hold the glock. Note that although these holders are 
dequeued-on-demand, they are never "uninitted" as part of the process. 
Nor must they ever be, since they may be on another process's heap.

This differs from the normal glock demote process in which the demote 
bit is set on ("requesting" the glock be demoted) but still needs to 
block until the holder does its actual dequeue.

>> Processes that allow a glock holder to be taken away indicate this by
>> calling gfs2_holder_allow_demote().  When they need the glock again,
>> they call gfs2_holder_disallow_demote() and then they check if the
>> holder is still queued: if it is, they're still holding the glock; if
>> it
>> isn't, they need to re-acquire the glock.
>>
>> This allows processes to hang on to locks that could become part of a
>> cyclic locking dependency.  The locks will be given up when a (rare)
>> conflicting locking request occurs, and don't need to be given up
>> prematurely.
> This seems backwards to me. We already have the glock layer cache the
> locks until they are required by another node. We also have the min
> hold time to make sure that we don't bounce locks too much. So what is
> the problem that you are trying to solve here I wonder?

Again, this is simply allowing premption of lenghy/low-priority holders 
whereas the normal demote process will only demote when the glock is 
dequeued after this potentially very-long period of time.

The minimum hold time solves a different problem, and Andreas and I 
talked just yesterday about possibly revisiting how that all works. The 
problem with minimum hold time is that in many cases the glock state 
machine does not want to grant new holders if the demote bit is on, so 
it ends up wasting more time than solving the actual problem.
But that's another problem for another day.

Regards,

Bob Peterson


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  reply	other threads:[~2021-08-20 13:11 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-19 19:40 [Ocfs2-devel] [PATCH v6 00/19] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 01/19] iov_iter: Fix iov_iter_get_pages{, _alloc} page fault return value Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 02/19] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 03/19] Turn fault_in_pages_{readable, writeable} into fault_in_{readable, writeable} Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 04/19] Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 06/19] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 07/19] gfs2: Clean up function may_grant Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 08/19] gfs2: Eliminate vestigial HIF_FIRST Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 09/19] gfs2: Remove redundant check from gfs2_glock_dq Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion Andreas Gruenbacher
2021-08-20  9:35   ` [Ocfs2-devel] [Cluster-devel] " Steven Whitehouse
2021-08-20 13:11     ` Bob Peterson [this message]
2021-08-20 13:41       ` Steven Whitehouse
2021-08-20 15:22       ` Andreas Gruenbacher
2021-08-23  8:14         ` Steven Whitehouse
2021-08-23 15:18           ` Andreas Gruenbacher
2021-08-23 16:05             ` Matthew Wilcox
2021-08-23 16:36               ` Bob Peterson
2021-08-23 19:12               ` Andreas Gruenbacher
2021-08-24  7:59             ` Steven Whitehouse
2021-08-20 13:17     ` Andreas Gruenbacher
2021-08-20 13:47       ` Steven Whitehouse
2021-08-20 14:43         ` Andreas Grünbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 11/19] gfs2: Move the inode glock locking to gfs2_file_buffered_write Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 12/19] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 13/19] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 14/19] iomap: Support partial direct I/O on user copy failures Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 15/19] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 16/19] gup: Introduce FOLL_NOFAULT flag to disable page faults Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 17/19] iov_iter: Introduce nofault " Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 18/19] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 19/19] gfs2: Eliminate ip->i_gh Andreas Gruenbacher

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