From: "Paul E. McKenney" <paulmck@kernel.org>
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Matthew Wilcox <willy@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Al Viro <viro@kernel.org>, Luis Chamberlain <mcgrof@kernel.org>,
lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org,
linux-mm <linux-mm@kvack.org>,
Daniel Gomez <da.gomez@samsung.com>,
Pankaj Raghav <p.raghav@samsung.com>,
Jens Axboe <axboe@kernel.dk>, Dave Chinner <david@fromorbit.com>,
Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO
Date: Tue, 27 Feb 2024 08:06:52 -0800 [thread overview]
Message-ID: <3c16052d-8e4c-4af7-ae82-f47ee058a884@paulmck-laptop> (raw)
In-Reply-To: <qclzh7gjlnuagsjiqemwvfnkxca2345zxansc7x463bguhsmm2@zl2cwv5fh5sv>
On Tue, Feb 27, 2024 at 10:52:51AM -0500, Kent Overstreet wrote:
> On Tue, Feb 27, 2024 at 07:32:32AM -0800, Paul E. McKenney wrote:
> > I could simply use the same general approach that I use within RCU
> > itself, which currently has absolutely no idea how much memory (if any)
> > that each callback will free. Especially given that some callbacks
> > free groups of memory blocks, while other free nothing. ;-)
> >
> > Alternatively, we could gather statistics on the amount of memory freed
> > by each callback and use that as an estimate.
> >
> > But we should instead step back and ask exactly what we are trying to
> > accomplish here, which just might be what Dave Chinner was getting at.
> >
> > At a ridiculously high level, reclaim is looking for memory to free.
> > Some read-only memory can often be dropped immediately on the grounds
> > that its data can be read back in if needed. Other memory can only be
> > dropped after being written out, which involves a delay. There are of
> > course many other complications, but this will do for a start.
> >
> > So, where does RCU fit in?
> >
> > RCU fits in between the two. With memory awaiting RCU, there is no need
> > to write anything out, but there is a delay. As such, memory waiting
> > for an RCU grace period is similar to memory that is to be reclaimed
> > after its I/O completes.
> >
> > One complication, and a complication that we are considering exploiting,
> > is that, unlike reclaimable memory waiting for I/O, we could often
> > (but not always) have some control over how quickly RCU's grace periods
> > complete. And we already do this programmatically by using the choice
> > between sychronize_rcu() and synchronize_rcu_expedited(). The question
> > is whether we should expedite normal RCU grace periods during reclaim,
> > and if so, under what conditions.
> >
> > You identified one potential condition, namely the amount of memory
> > waiting to be reclaimed. One complication with this approach is that RCU
> > has no idea how much memory each callback represents, and for call_rcu(),
> > there is no way for it to find out. For kfree_rcu(), there are ways,
> > but as you know, I am questioning whether those ways are reasonable from
> > a performance perspective. But even if they are, we would be accepting
> > more error from the memory waiting via call_rcu() than we would be
> > accepting if we just counted blocks instead of bytes for kfree_rcu().
>
> You're _way_ overcomplicating this.
Sorry, but no.
Please read the remainder of my prior email carefully.
Thanx, Paul
> The relevant thing to consider is the relative cost of __ksize() and
> kfree_rcu(). __ksize() is already pretty cheap, and with slab gone and
> space available in struct slab we can get it down to a single load.
>
> > Let me reiterate that: The estimation error that you are objecting to
> > for kfree_rcu() is completely and utterly unavoidable for call_rcu().
>
> hardly, callsites manually freeing memory manually after an RCU grace
> period can do the accounting manually - if they're hot enough to matter,
> most aren.t
>
> and with memory allocation profiling coming, which also tracks # of
> allocations, we'll also have an easy way to spot those.
next prev parent reply other threads:[~2024-02-27 16:07 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-23 23:59 [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO Luis Chamberlain
2024-02-24 4:12 ` Matthew Wilcox
2024-02-24 17:31 ` Linus Torvalds
2024-02-24 18:13 ` Matthew Wilcox
2024-02-24 18:24 ` Linus Torvalds
2024-02-24 18:20 ` Linus Torvalds
2024-02-24 19:11 ` Linus Torvalds
2024-02-24 21:42 ` Theodore Ts'o
2024-02-24 22:57 ` Chris Mason
2024-02-24 23:40 ` Linus Torvalds
2024-05-10 23:57 ` Luis Chamberlain
2024-02-25 5:18 ` Kent Overstreet
2024-02-25 6:04 ` Kent Overstreet
2024-02-25 13:10 ` Matthew Wilcox
2024-02-25 17:03 ` Linus Torvalds
2024-02-25 21:14 ` Matthew Wilcox
2024-02-25 23:45 ` Linus Torvalds
2024-02-26 1:02 ` Kent Overstreet
2024-02-26 1:32 ` Linus Torvalds
2024-02-26 1:58 ` Kent Overstreet
2024-02-26 2:06 ` Kent Overstreet
2024-02-26 2:34 ` Linus Torvalds
2024-02-26 2:50 ` Al Viro
2024-02-26 17:17 ` Linus Torvalds
2024-02-26 21:07 ` Matthew Wilcox
2024-02-26 21:17 ` Kent Overstreet
2024-02-26 21:19 ` Kent Overstreet
2024-02-26 21:55 ` Paul E. McKenney
2024-02-26 23:29 ` Kent Overstreet
2024-02-27 0:05 ` Paul E. McKenney
2024-02-27 0:29 ` Kent Overstreet
2024-02-27 0:55 ` Paul E. McKenney
2024-02-27 1:08 ` Kent Overstreet
2024-02-27 5:17 ` Paul E. McKenney
2024-02-27 6:21 ` Kent Overstreet
2024-02-27 15:32 ` Paul E. McKenney
2024-02-27 15:52 ` Kent Overstreet
2024-02-27 16:06 ` Paul E. McKenney [this message]
2024-02-27 15:54 ` Matthew Wilcox
2024-02-27 16:21 ` Paul E. McKenney
2024-02-27 16:34 ` Kent Overstreet
2024-02-27 17:58 ` Paul E. McKenney
2024-02-28 23:55 ` Kent Overstreet
2024-02-29 19:42 ` Paul E. McKenney
2024-02-29 20:51 ` Kent Overstreet
2024-03-05 2:19 ` Paul E. McKenney
2024-02-27 0:43 ` Dave Chinner
2024-02-26 22:46 ` Linus Torvalds
2024-02-26 23:48 ` Linus Torvalds
2024-02-27 7:21 ` Kent Overstreet
2024-02-27 15:39 ` Matthew Wilcox
2024-02-27 15:54 ` Kent Overstreet
2024-02-27 16:34 ` Linus Torvalds
2024-02-27 16:47 ` Kent Overstreet
2024-02-27 17:07 ` Linus Torvalds
2024-02-27 17:20 ` Kent Overstreet
2024-02-27 18:02 ` Linus Torvalds
2024-05-14 11:52 ` Luis Chamberlain
2024-05-14 16:04 ` Linus Torvalds
2024-02-25 21:29 ` Kent Overstreet
2024-02-25 17:32 ` Kent Overstreet
2024-02-24 17:55 ` Luis Chamberlain
2024-02-25 5:24 ` Kent Overstreet
2024-02-26 12:22 ` Dave Chinner
2024-02-27 10:07 ` Kent Overstreet
2024-02-27 14:08 ` Luis Chamberlain
2024-02-27 14:57 ` Kent Overstreet
2024-02-27 22:13 ` Dave Chinner
2024-02-27 22:21 ` Kent Overstreet
2024-02-27 22:42 ` Dave Chinner
2024-02-28 7:48 ` [Lsf-pc] " Amir Goldstein
2024-02-28 14:01 ` Chris Mason
2024-02-29 0:25 ` Dave Chinner
2024-02-29 0:57 ` Kent Overstreet
2024-03-04 0:46 ` Dave Chinner
2024-02-27 22:46 ` Linus Torvalds
2024-02-27 23:00 ` Linus Torvalds
2024-02-28 2:22 ` Kent Overstreet
2024-02-28 3:00 ` Matthew Wilcox
2024-02-28 4:22 ` Matthew Wilcox
2024-02-28 17:34 ` Kent Overstreet
2024-02-28 18:04 ` Matthew Wilcox
2024-02-28 18:18 ` Kent Overstreet
2024-02-28 19:09 ` Linus Torvalds
2024-02-28 19:29 ` Kent Overstreet
2024-02-28 20:17 ` Linus Torvalds
2024-02-28 23:21 ` Kent Overstreet
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