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[209.132.180.67]) by mx.google.com with ESMTP id y17si408614oix.228.2020.03.31.21.54.04; Tue, 31 Mar 2020 21:54:17 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731683AbgDAEwm (ORCPT + 99 others); Wed, 1 Apr 2020 00:52:42 -0400 Received: from mx2.suse.de ([195.135.220.15]:45860 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726125AbgDAEwl (ORCPT ); Wed, 1 Apr 2020 00:52:41 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 649B6AEEA; Wed, 1 Apr 2020 04:52:38 +0000 (UTC) From: NeilBrown To: Joel Fernandes Date: Wed, 01 Apr 2020 15:52:25 +1100 Cc: Michal Hocko , 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 , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , Steven Rostedt Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern In-Reply-To: <20200401032555.GA175966@google.com> References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331145806.GB236678@google.com> <20200331153450.GM30449@dhcp22.suse.cz> <20200331160117.GA170994@google.com> <877dz0yxoa.fsf@notabene.neil.brown.name> <20200401032555.GA175966@google.com> Message-ID: <874ku3zu2e.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Tue, Mar 31 2020, Joel Fernandes wrote: > On Wed, Apr 01, 2020 at 09:19:49AM +1100, NeilBrown wrote: >> On Tue, Mar 31 2020, Joel Fernandes wrote: >>=20 >> > On Tue, Mar 31, 2020 at 05:34:50PM +0200, Michal Hocko wrote: >> >> On Tue 31-03-20 10:58:06, Joel Fernandes wrote: >> >> [...] >> >> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c >> >> > > index 4be763355c9fb..965deefffdd58 100644 >> >> > > --- a/kernel/rcu/tree.c >> >> > > +++ b/kernel/rcu/tree.c >> >> > > @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_h= ead_to_object(void *obj) >> >> > >=20=20 >> >> > > if (!ptr) >> >> > > ptr =3D kmalloc(sizeof(unsigned long *) + >> >> > > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN); >> >> > > + sizeof(struct rcu_head), GFP_MEMALLOC); >> >> >=20 >> >> > Just to add, the main requirements here are: >> >> > 1. Allocation should be bounded in time. >> >> > 2. Allocation should try hard (possibly tapping into reserves) >> >> > 3. Sleeping is Ok but should not affect the time bound. >> >>=20 >> >>=20 >> >> __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to >> >> memory reserves regarless of the sleeping status. >> >>=20 >> >> Using __GFP_MEMALLOC is quite dangerous because it can deplete _all_ = the >> >> memory. What does prevent the above code path to do that? >> > >> > Can you suggest what prevents other users of GFP_MEMALLOC from doing t= hat >> > also? That's the whole point of having a reserve, in normal usage no o= ne will >> > use it, but some times you need to use it. Keep in mind this is not a = common >> > case in this code here, this is triggered only if earlier allocation a= ttempts >> > failed. Only *then* we try with GFP_MEMALLOC with promises to free add= itional >> > memory soon. >>=20 >> I think that "soon" is the key point. Users of __GFP_MEMALLOC certainly >> must be working to free other memory, that other memory needs to be freed >> "soon". In particular - sooner than all the reserve is exhausted. This >> can require rate-limiting. If one allocation can result in one page >> being freed, that is good and it is probably OK to have 1000 allocations >> resulting in 1000 pages being freed soon. But 10 million allocation to >> gain 10 million pages is not such a good thing and shouldn't be needed. >> Once those first 1000 pages have been freed, you won't need >> __GFP_MEMALLOC allocations any more, and you must be prepare to wait for >> them. >>=20 >> So where does the rate-limiting happen in your proposal? A GP can be >> multiple milliseconds, which is time for lots of memory to be allocated >> and for rcu-free queues to grow quite large. >>=20 >> You mention a possible fall-back of calling synchronize_rcu(). I think >> that needs to be a fallback that happens well before __GFP_MEMALLOC is >> exhausted. You need to choose some maximum amount that you will >> allocate, then use synchronize_rcu() (or probably the _expedited >> version) after that. The pool of reserves are certainly there for you >> to use, but not for you to exhaust. >>=20 >> If you have your own rate-limiting, then I think __GFP_MEMALLOC is >> probably OK, and also you *don't* want the memalloc to wait. If memory >> cannot be allocated immediately, you need to use your own fallback. > > Thanks a lot for explaining in detail, the RFC patch has served its purpo= se > well ;-) > > On discussing with RCU comrades, we agreed to not use GFP_MEMALLOC. But > instead pre-allocate a cache (we do have a cache but it is not yet > pre-allocated, just allocated on demand). > > About the rate limiting, we would fallback to synchronize_rcu() instead of > sleeping in case of trobule. However I would like to add a warning if we = ever > hit the troublesome path mainly because that means we depleted the > pre-allocated cache and perhaps the user should switch to adding an rcu_h= ead > in their structure to reduce latency. I'm adding that warning to my tree: If this warning is only interesting to developers, I think you should only show it to developers, not to end-users. i.e. protect it with CONFIG_DEBUG_RCU or something like that. NeilBrown > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 4be763355c9fb..6172e6296dd7d 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -110,6 +110,10 @@ module_param(rcu_fanout_exact, bool, 0444); > static int rcu_fanout_leaf =3D RCU_FANOUT_LEAF; > module_param(rcu_fanout_leaf, int, 0444); > int rcu_num_lvls __read_mostly =3D RCU_NUM_LVLS; > +/* Silence the kvfree_rcu() complaint (warning) that it blocks */ > +int rcu_kfree_nowarn; > +module_param(rcu_kfree_nowarn, int, 0444); > + > /* Number of rcu_nodes at specified level. */ > int num_rcu_lvl[] =3D NUM_RCU_LVL_INIT; > int rcu_num_nodes __read_mostly =3D NUM_RCU_NODES; /* Total # rcu_nodes = in use. */ > @@ -3266,6 +3270,12 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_ca= llback_t func) > * state. > */ > if (!success) { > + /* > + * Please embed an rcu_head and pass it along if you hit this > + * warning. Doing so would avoid long kfree_rcu() latencies. > + */ > + if (!rcu_kfree_nowarn) > + WARN_ON_ONCE(1); > debug_rcu_head_unqueue(ptr); > synchronize_rcu(); > kvfree(ptr); --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAl6EHgoACgkQOeye3VZi gbkm2w//fdZ+L31EJT6f/eM53SBbDzDAxkF0FocZ0ScJk4OLbo6kvFWVE8YToX0N dDoKJMy7QALVFa+sLy1LSuy7h2MxSHUvRiGXVeDnzJXDjQiz4KWD4F1JzVacaICD 5cIQd8KaDNQjC92wWKHFnhtCoTSY/KAb45J96jdst+rWvRsSyRx8chXPs/Oljg+e 15W9ph8o346FvaY0q4AuA6feK2F/OxENv5+b89Y9kWd+UlTpLQWMj0nRkygK7igM IgA/FvcqT0KOtkc3o8lTSKcci0eTbI+tNLNbwNanwyQ0tYKJCCdXfTGa8cWb9lhl AOB3sxIkZbQl16MtH1WzCBZL1B4tDfjan8YxWCvi2lX5iaBQz/zUKQr/bth0hf93 Z/0dQNnMjjonICewh7xzjaezlkgu7GSfnOFiXQpHIhCA1qgFLGrmq/SMlnSi4XCV l2zdWUmvAIJBQtCmT05jCBqbUWobvJm7otMuBv9hygxRGOWC+fkEUnvLEAZSdv2K srbNgwsS09D3qL+KZ9loOgFzn9glJqOb4d2paEwn/4Z2G3MjqoM6yzkcJvYrKUBg KHbnYHhLtVrRzvb8vE0WS3Y55MmGDhMHOjbWcWewiPGdIGUGSKeVx38TmukZ+DmY vkUIIAD6txSrh+0pwG2+ElNvH4Q6mZmuMgteFq7dhuJdyk3dcUQ= =JV7+ -----END PGP SIGNATURE----- --=-=-=--