From: "Paul E. McKenney" <paulmck@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: 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 11:26:15 -0700 [thread overview]
Message-ID: <20200401182615.GE19865@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200401181601.GA4042@pc636>
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.
Thanx, Paul
> > If not, the current per-CPU pre-allocated cache is a better choice in the
> > near term.
> >
> OK. I see your point.
>
> Thank you for your comments and view :)
>
> --
> Vlad Rezki
next prev parent reply other threads:[~2020-04-01 18:26 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 [this message]
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
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
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