Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion

[Steven Whitehouse]

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@xxxxxxxxxx> 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@xxxxxxxxxx
> > > >
> > > 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.