ocfs2-devel.oss.oracle.com archive mirror
 help / color / mirror / Atom feed
From: Steven Whitehouse <swhiteho@redhat.com>
To: Andreas Gruenbacher <agruenba@redhat.com>
Cc: cluster-devel <cluster-devel@redhat.com>, Jan Kara <jack@suse.cz>,
	LKML <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Bob Peterson <rpeterso@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.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: Tue, 24 Aug 2021 08:59:24 +0100	[thread overview]
Message-ID: <2c0fbb32e7668844f148b12cb4711abc76f50fe4.camel@redhat.com> (raw)
In-Reply-To: <CAHc6FU5uHJSXD+CQk3W9BfZmnBCd+fqHt4Bd+=uVH18rnYCPLg@mail.gmail.com>

Hi,

On Mon, 2021-08-23 at 17:18 +0200, Andreas Gruenbacher wrote:
> On Mon, Aug 23, 2021 at 10:14 AM Steven Whitehouse <
> swhiteho@redhat.com> wrote:
> > On Fri, 2021-08-20 at 17:22 +0200, Andreas Gruenbacher wrote:
> > > On Fri, Aug 20, 2021 at 3:11 PM Bob Peterson <rpeterso@redhat.com
> > > >
> > > wrote:
> > [snip]
> > > > 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.
> > > 
> > > Consider a process that allocates a somewhat large buffer and
> > > reads
> > > into it in chunks that are not page aligned. The buffer initially
> > > won't be faulted in, so we fault in the first chunk and write
> > > into
> > > it.
> > > Then, when reading the second chunk, we find that the first page
> > > of
> > > the second chunk is already present. We fill it, set the
> > > HIF_MAY_DEMOTE flag, fault in more pages, and clear the
> > > HIF_MAY_DEMOTE
> > > flag. If we then still have the glock (which is very likely), we
> > > resume the read. Otherwise, we return a short result.
> > > 
> > > Thanks,
> > > Andreas
> > > 
> > 
> > If the goal here is just to allow the glock to be held for a longer
> > period of time, but with occasional interruptions to prevent
> > starvation, then we have a potential model for this. There is
> > cond_resched_lock() which does this for spin locks.
> 
> This isn't an appropriate model for what I'm trying to achieve here.
> In the cond_resched case, we know at the time of the cond_resched
> call
> whether or not we want to schedule. If we do, we want to drop the
> spin
> lock, schedule, and then re-acquire the spin lock. In the case we're
> looking at here, we want to fault in user pages. There is no way of
> knowing beforehand if the glock we're currently holding will have to
> be dropped to achieve that. In fact, it will almost never have to be
> dropped. But if it does, we need to drop it straight away to allow
> the
> conflicting locking request to succeed.
> 
> Have a look at how the patch queue uses gfs2_holder_allow_demote()
> and
> gfs2_holder_disallow_demote():
> 
> https://listman.redhat.com/archives/cluster-devel/2021-August/msg00128.html
> https://listman.redhat.com/archives/cluster-devel/2021-August/msg00134.html
> 
> Thanks,
> Andreas
> 

Ah, now I see! Apologies if I've misunderstood the intent here,
initially. It is complicated and not so obvious - at least to me!

You've added a lot of context to this patch in your various replies on
this thread. The original patch description explains how the feature is
implemented, but doesn't really touch on why - that is left to the
other patches that you pointed to above. A short paragraph or two on
the "why" side of things added to the patch description would be
helpful I think.

Your message on Friday (20 Aug 2021 15:17:41 +0200 (20/08/21 14:17:41))
has a good explanation in the second part of it, which with what you've
said above (or similar) would be a good basis.

Apologies again for not understanding what is going on,

Steve.



_______________________________________________
Ocfs2-devel mailing list
Ocfs2-devel@oss.oracle.com
https://oss.oracle.com/mailman/listinfo/ocfs2-devel

  parent reply	other threads:[~2021-08-24  8:03 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
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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=2c0fbb32e7668844f148b12cb4711abc76f50fe4.camel@redhat.com \
    --to=swhiteho@redhat.com \
    --cc=agruenba@redhat.com \
    --cc=cluster-devel@redhat.com \
    --cc=hch@infradead.org \
    --cc=jack@suse.cz \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=ocfs2-devel@oss.oracle.com \
    --cc=rpeterso@redhat.com \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@zeniv.linux.org.uk \
    --subject='Re: [Ocfs2-devel] [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).