From: Uladzislau Rezki <urezki@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org,
neilb@suse.com, vbabka@suse.cz, mgorman@suse.de,
Andrew Morton <akpm@linux-foundation.org>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern
Date: Wed, 1 Apr 2020 21:35:24 +0200 [thread overview]
Message-ID: <20200401193524.GA6821@pc636> (raw)
In-Reply-To: <20200401193405.GH19865@paulmck-ThinkPad-P72>
On Wed, Apr 01, 2020 at 12:34:05PM -0700, Paul E. McKenney wrote:
> On Wed, Apr 01, 2020 at 09:05:48PM +0200, Uladzislau Rezki wrote:
> > On Wed, Apr 01, 2020 at 11:54:39AM -0700, Paul E. McKenney wrote:
> > > On Wed, Apr 01, 2020 at 08:37:45PM +0200, Uladzislau Rezki wrote:
> > > > On Wed, Apr 01, 2020 at 11:26:15AM -0700, Paul E. McKenney wrote:
> > > > > On Wed, Apr 01, 2020 at 08:16:01PM +0200, Uladzislau Rezki wrote:
> > > > > > > > >
> > > > > > > > > Right. Per discussion with Paul, we discussed that it is better if we
> > > > > > > > > pre-allocate N number of array blocks per-CPU and use it for the cache.
> > > > > > > > > Default for N being 1 and tunable with a boot parameter. I agree with this.
> > > > > > > > >
> > > > > > > > As discussed before, we can make use of memory pool API for such
> > > > > > > > purpose. But i am not sure if it should be one pool per CPU or
> > > > > > > > one pool per NR_CPUS, that would contain NR_CPUS * N pre-allocated
> > > > > > > > blocks.
> > > > > > >
> > > > > > > There are advantages and disadvantages either way. The advantage of the
> > > > > > > per-CPU pool is that you don't have to worry about something like lock
> > > > > > > contention causing even more pain during an OOM event. One potential
> > > > > > > problem wtih the per-CPU pool can happen when callbacks are offloaded,
> > > > > > > in which case the CPUs needing the memory might never be getting it,
> > > > > > > because in the offloaded case (RCU_NOCB_CPU=y) the CPU posting callbacks
> > > > > > > might never be invoking them.
> > > > > > >
> > > > > > > But from what I know now, systems built with CONFIG_RCU_NOCB_CPU=y
> > > > > > > either don't have heavy callback loads (HPC systems) or are carefully
> > > > > > > configured (real-time systems). Plus large systems would probably end
> > > > > > > up needing something pretty close to a slab allocator to keep from dying
> > > > > > > from lock contention, and it is hard to justify that level of complexity
> > > > > > > at this point.
> > > > > > >
> > > > > > > Or is there some way to mark a specific slab allocator instance as being
> > > > > > > able to keep some amount of memory no matter what the OOM conditions are?
> > > > > > > If not, the current per-CPU pre-allocated cache is a better choice in the
> > > > > > > near term.
> > > > > > >
> > > > > > As for mempool API:
> > > > > >
> > > > > > mempool_alloc() just tries to make regular allocation taking into
> > > > > > account passed gfp_t bitmask. If it fails due to memory pressure,
> > > > > > it uses reserved preallocated pool that consists of number of
> > > > > > desirable elements(preallocated when a pool is created).
> > > > > >
> > > > > > mempoll_free() returns an element to to pool, if it detects that
> > > > > > current reserved elements are lower then minimum allowed elements,
> > > > > > it will add an element to reserved pool, i.e. refill it. Otherwise
> > > > > > just call kfree() or whatever we define as "element-freeing function."
> > > > >
> > > > > Unless I am missing something, mempool_alloc() acquires a per-mempool
> > > > > lock on each invocation under OOM conditions. For our purposes, this
> > > > > is essentially a global lock. This will not be at all acceptable on a
> > > > > large system.
> > > > >
> > > > It uses pool->lock to access to reserved objects, so if we have one memory
> > > > pool per one CPU then it would be serialized.
> > >
> > > I am having difficulty parsing your sentence. It looks like your thought
> > > is to invoke mempool_create() for each CPU, so that the locking would be
> > > on a per-CPU basis, as in 128 invocations of mempool_init() on a system
> > > having 128 hardware threads. Is that your intent?
> > >
> > In order to serialize it, you need to have it per CPU. So if you have 128
> > cpus, it means:
> >
> > <snip>
> > for_each_possible_cpu(...)
> > cpu_pool = mempool_create();
> > <snip>
> >
> > but please keep in mind that it is not my intention, but i had a though
> > about mempool API. Because it has pre-reserve logic inside.
>
> OK, fair point on use of mempool API, but my guess is that extending
> the current kfree_rcu() logic will be simpler.
>
Agree :)
--
Vlad Rezki
next prev parent reply other threads:[~2020-04-01 19:35 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-31 13:16 [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Joel Fernandes (Google)
2020-03-31 14:04 ` Uladzislau Rezki
2020-03-31 15:09 ` Joel Fernandes
2020-03-31 16:01 ` Uladzislau Rezki
2020-03-31 17:02 ` Uladzislau Rezki
2020-03-31 17:49 ` Paul E. McKenney
2020-03-31 18:30 ` Joel Fernandes
2020-04-01 12:25 ` Uladzislau Rezki
2020-04-01 13:47 ` Paul E. McKenney
2020-04-01 18:16 ` Uladzislau Rezki
2020-04-01 18:26 ` Paul E. McKenney
2020-04-01 18:37 ` Uladzislau Rezki
2020-04-01 18:54 ` Paul E. McKenney
2020-04-01 19:05 ` Uladzislau Rezki
2020-04-01 19:34 ` Paul E. McKenney
2020-04-01 19:35 ` Uladzislau Rezki [this message]
2020-03-31 14:58 ` Joel Fernandes
2020-03-31 15:34 ` Michal Hocko
2020-03-31 16:01 ` Joel Fernandes
2020-03-31 22:19 ` NeilBrown
2020-04-01 3:25 ` Joel Fernandes
2020-04-01 4:52 ` NeilBrown
2020-04-01 7:23 ` Michal Hocko
2020-04-01 11:14 ` joel
2020-04-01 12:05 ` Michal Hocko
2020-04-01 13:14 ` Mel Gorman
2020-04-01 14:45 ` Joel Fernandes
2020-03-31 16:12 ` Uladzislau Rezki
2020-04-01 7:09 ` Michal Hocko
2020-04-01 12:32 ` Uladzislau Rezki
2020-04-01 12:55 ` Michal Hocko
2020-04-01 13:08 ` Uladzislau Rezki
2020-04-01 13:15 ` Michal Hocko
2020-04-01 13:22 ` Uladzislau Rezki
2020-04-01 15:28 ` Michal Hocko
2020-04-01 15:46 ` Uladzislau Rezki
2020-04-01 15:57 ` Paul E. McKenney
2020-04-01 16:10 ` Michal Hocko
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=20200401193524.GA6821@pc636 \
--to=urezki@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=jiangshanlai@gmail.com \
--cc=joel@joelfernandes.org \
--cc=josh@joshtriplett.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mgorman@suse.de \
--cc=neilb@suse.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=rcu@vger.kernel.org \
--cc=rostedt@goodmis.org \
--cc=vbabka@suse.cz \
--cc=willy@infradead.org \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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).