From: Uladzislau Rezki <urezki@gmail.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
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>,
"Paul E. McKenney" <paulmck@kernel.org>,
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 14:25:50 +0200 [thread overview]
Message-ID: <20200401122550.GA32593@pc636> (raw)
In-Reply-To: <20200331183000.GD236678@google.com>
> > I think there should be GFP_ATOMIC used, because it has more chance to
> > return memory then GFP_NOWAIT. I see that Michal has same view on it.
>
> I don't think so because GFP_ATOMIC implies GFP_NOWAIT. I am Ok with keeping
> the GFP_ATOMIC as it is btw. Paul mentioned he prefers this. I agree with
> that as well.
>
GFP_ATOMIC can access to reserved memory whereas GFP_NOWAIT is not
eligible to do so. So there is difference between them :)
> > >
> > > Yes, the benefit of the trace/warning is that the user can switch to a
> > > non-headless API and avoid the synchronize_rcu(), that would help them get
> > > faster kfree_rcu() performance instead of having silent slowdowns.
> > >
> > Agree. What about just adding WARN_ON_ONCE()? I am just thinking if it
> > could be harmful or not.
>
> You mean WARN_ON_ONCE() before the synchronize_rcu() right? We could do that.
> Paul mentioned to me he prefers if this new warning can be turned off with a
> boot parameter since some future user may prefer no warning. I also agree.
>
Yes, we can add it before doing synchronize_rcu(). WARN_ON_ONCE() will
emit only once the warning. I think that would be enough to pay an
attention.
>
> If we add this then we can keep your __GFP_NOWARN flag with no additional GFP
> flag changes.
>
We can also add __GFP_RETRY_MAYFAIL to GFP_ATOMIC to make it more tight.
Basically your patch can be modified just adding that.
> > > It also tells us whether the headless API is worth it in the long run, I
> > > think it is worth it because we will likely never hit the synchronize_rcu()
> > > failsafe. But if we hit it a lot, at least it wont happen silently.
> > >
> > Agree.
> >
> > > Paul was concerned about following scenario with hitting synchronize_rcu():
> > > 1. Consider a system under memory pressure.
> > > 2. Consider some other subsystem X depending on another system Y which uses
> > > kfree_rcu(). If Y doesn't complete the operation in time, X accumulates
> > > more memory.
> > > 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down.
> > > This causes X to further allocate memory, further causing a chain
> > > reaction.
> > > Paul, please correct me if I'm wrong.
> > >
> > I see your point and agree that in theory it can happen. So, we should
> > make it more tight when it comes to rcu_head attachment logic.
>
> 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.
> In current code, we have 1 cache page per CPU, but this is allocated only on
> the first kvfree_rcu() request. So we could change this behavior as well to
> make it pre-allocated.
>
> Does this all sound good to you?
>
I think that makes sense :)
--
Vlad Rezki
next prev parent reply other threads:[~2020-04-01 12:26 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 [this message]
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
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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