Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

From: Uladzislau Rezki <urezki@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Michal Hocko <mhocko@suse.com>,
	LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Joel Fernandes <joel@joelfernandes.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
	Mel Gorman <mgorman@suse.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Date: Thu, 24 Sep 2020 17:21:12 +0200	[thread overview]
Message-ID: <20200924152112.GB19013@pc636> (raw)
In-Reply-To: <20200924111907.GE2628@hirez.programming.kicks-ass.net>

On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > The key point is "enough". We need pages to make a) fast progress b) support
> > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
> > on scheduler latency and vague pre-allocated number of pages, it might
> > be not enough what would require to refill it more and more or we can overshoot
> > that would lead to memory overhead. So we have here timing issues and
> > not accurate model. IMHO.
> 
> I'm firmly opposed to the single argument kvfree_rcu() idea, that's
> requiring memory to free memory.
> 
Hmm.. The problem is there is a demand in it:

Please have a look at places in the kernel where people do not
embed the rcu_head into their stuctures and do like:

<snip>
    synchronize_rcu();
    kfree(p);
<snip>

<snip>
urezki@pc638:~/data/coding/linux-rcu.git$ find ./ -name "*.c" | xargs grep -C 1 -rn "synchronize_rcu" | grep kfree
./fs/nfs/sysfs.c-113-           kfree(old);
./fs/ext4/super.c-1708- kfree(old_qname);
./kernel/trace/ftrace.c-5079-                   kfree(direct);
./kernel/trace/ftrace.c-5156-                   kfree(direct);
./kernel/trace/trace_probe.c-1087-      kfree(link);
./kernel/module.c-3910- kfree(mod->args);
./net/core/sysctl_net_core.c-143-                               kfree(cur);
./arch/x86/mm/mmio-mod.c-314-           kfree(found_trace);
./drivers/mfd/dln2.c-183-               kfree(i);
./drivers/block/drbd/drbd_state.c-2074-         kfree(old_conf);
./drivers/block/drbd/drbd_nl.c-1689-    kfree(old_disk_conf);
./drivers/block/drbd/drbd_nl.c-2522-    kfree(old_net_conf);
./drivers/block/drbd/drbd_nl.c-2935-            kfree(old_disk_conf);
./drivers/block/drbd/drbd_receiver.c-3805-      kfree(old_net_conf);
./drivers/block/drbd/drbd_receiver.c-4177-                      kfree(old_disk_conf);
./drivers/ipack/carriers/tpci200.c-189- kfree(slot_irq);
./drivers/crypto/nx/nx-842-pseries.c-1010-      kfree(old_devdata);
./drivers/net/ethernet/myricom/myri10ge/myri10ge.c-3583-        kfree(mgp->ss);
./drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c:286:       synchronize_rcu(); /* before kfree(flow) */
./drivers/net/ethernet/mellanox/mlxsw/core.c-1574-      kfree(rxl_item);
./drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c-6642- kfree(adapter->mbox_log);
./drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c-6644- kfree(adapter);
./drivers/infiniband/hw/hfi1/sdma.c-1337-       kfree(dd->per_sdma);
./drivers/infiniband/core/device.c:2164:                         * synchronize_rcu before the netdev is kfreed, so we
./drivers/misc/vmw_vmci/vmci_context.c-692-             kfree(notifier);
./drivers/misc/vmw_vmci/vmci_event.c-213-       kfree(s);
./drivers/staging/fwserial/fwserial.c-2122-     kfree(peer);
urezki@pc638:~/data/coding/linux-rcu.git$
<snip>

--
Vlad Rezki