From: Uladzislau Rezki <email@example.com> To: Michal Hocko <firstname.lastname@example.org> Cc: Uladzislau Rezki <email@example.com>, LKML <firstname.lastname@example.org>, RCU <email@example.com>, firstname.lastname@example.org, Andrew Morton <email@example.com>, Vlastimil Babka <firstname.lastname@example.org>, "Paul E . McKenney" <email@example.com>, Matthew Wilcox <firstname.lastname@example.org>, "Theodore Y . Ts'o" <email@example.com>, Joel Fernandes <firstname.lastname@example.org>, Sebastian Andrzej Siewior <email@example.com>, Oleksiy Avramchenko <firstname.lastname@example.org> Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Date: Tue, 11 Aug 2020 11:18:07 +0200 [thread overview] Message-ID: <20200811091807.GA2634@pc636> (raw) In-Reply-To: <20200810192525.GG4773@dhcp22.suse.cz> On Mon, Aug 10, 2020 at 09:25:25PM +0200, Michal Hocko wrote: > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote: > > > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote: > > > [...] > > > > Limitations and concerns (Main part) > > > > ==================================== > > > > The current memmory-allocation interface presents to following > > > > difficulties that this patch is designed to overcome: > > > > > > > > a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep will > > > > complain about violation("BUG: Invalid wait context") of the > > > > nesting rules. It does the raw_spinlock vs. spinlock nesting > > > > checks, i.e. it is not legal to acquire a spinlock_t while > > > > holding a raw_spinlock_t. > > > > > > > > Internally the kfree_rcu() uses raw_spinlock_t(in rcu-dev branch) > > > > whereas the "page allocator" internally deals with spinlock_t to > > > > access to its zones. The code also can be broken from higher level > > > > of view: > > > > <snip> > > > > raw_spin_lock(&some_lock); > > > > kfree_rcu(some_pointer, some_field_offset); > > > > <snip> > > > > > > Is there any fundamental problem to make zone raw_spin_lock? > > > > > Good point. Converting a regular spinlock to the raw_* variant can solve > > an issue and to me it seems partly reasonable. Because there are other > > questions if we do it: > > > > a) what to do with kswapd and "wake-up path" that uses sleepable lock: > > wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait). > > If there is no RT friendly variant for waking up process from the atomic > context then we might need to special case this for the RT tree. > I do not see it in RT kernel. The waiting primitives, see the wait.c, use sleepable locks all over the file. > > b) How RT people reacts on it? I guess they will no be happy. > > zone->lock should be held for a very limited amount of time. > > > As i described before, calling the __get_free_page(0) with 0 as argument > > will solve the (a). How correctly is it? From my point of view the logic > > that bypass the wakeup path should be explicitly defined. > > gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which > doesn't wake up kswapd. So if the wakeup is a problem then this would be > a way to go. > What do you mean Michal? gfp_mask 0 != GFP_NOWAIT: #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM) it does wakeup of the kswapd. Or am i missing something? Please comment. If we are about to avoid the kswapd, should we define something special? #define GFP_NOWWAKE_KSWAPD 0 > > Or we can enter the allocator with (__GFP_HIGH|__GFP_ATOMIC) that bypass > > the __GFP_KSWAPD_RECLAIM as well. > > This would be an alternative which consumes memory reserves. Is this > really needed for the particular case? > No. That was just another example illustrating how to bypass the __GFP_KSWAPD_RECLAIM. > > > > Any thoughts here? Please comment. > > > > Having proposed flag will not heart RT latency and solve all concerns. > > > > > > b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t > > > > is converted into sleepable variant. Invoking the page allocator from > > > > atomic contexts leads to "BUG: scheduling while atomic". > > > > > > [...] > > > > > > > Proposal > > > > ======== > > > > 1) Make GFP_* that ensures that the allocator returns NULL rather > > > > than acquire its own spinlock_t. Having such flag will address a and b > > > > limitations described above. It will also make the kfree_rcu() code > > > > common for RT and regular kernel, more clean, less handling corner > > > > cases and reduce the code size. > > > > > > I do not think this is a good idea. Single purpose gfp flags that tend > > > to heavily depend on the current implementation of the page allocator > > > have turned out to be problematic. Users used to misunderstand their > > > meaning resulting in a lot of abuse which was not trivial to remove. > > > This flag seem to fall into exactly this sort of category. If there is a > > > problem in nesting then that should be addressed rather than a new flag > > > exported IMHO. If that is absolutely not possible for some reason then > > > we can try to figure out what to do but that really need a very strong > > > justification. > > > > > The problem that i see is we can not use the page allocator from atomic > > contexts, what is our case: > > > > <snip> > > local_irq_save(flags) or preempt_disable() or raw_spinlock(); > > __get_free_page(GFP_ATOMIC); > > <snip> > > > > So if we can convert the page allocator to raw_* lock it will be appreciated, > > at least from our side, IMHO, not from RT one. But as i stated above we need > > to sort raised questions out if converting is done. > > > > What is your view? > > To me it would make more sense to support atomic allocations also for > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really > work for atomic context in RT sounds subtle and wrong. > Same view on it. Thank you for your comments! -- Vlad Rezki
next prev parent reply other threads:[~2020-08-11 9:18 UTC|newest] Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-08-09 20:43 [RFC-PATCH 0/2] __GFP_NO_LOCKS Uladzislau Rezki (Sony) 2020-08-09 20:43 ` [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Uladzislau Rezki (Sony) 2020-08-10 12:31 ` Michal Hocko 2020-08-10 16:07 ` Uladzislau Rezki 2020-08-10 19:25 ` Michal Hocko 2020-08-11 8:19 ` Michal Hocko 2020-08-11 9:37 ` Uladzislau Rezki 2020-08-11 9:42 ` Uladzislau Rezki 2020-08-11 10:28 ` Michal Hocko 2020-08-11 10:45 ` Uladzislau Rezki 2020-08-11 10:26 ` Michal Hocko 2020-08-11 11:33 ` Uladzislau Rezki 2020-08-11 9:18 ` Uladzislau Rezki [this message] 2020-08-11 10:21 ` Michal Hocko 2020-08-11 11:10 ` Uladzislau Rezki 2020-08-11 14:44 ` Thomas Gleixner 2020-08-11 15:22 ` Thomas Gleixner 2020-08-12 11:38 ` Thomas Gleixner 2020-08-12 12:01 ` Uladzislau Rezki 2020-08-13 7:18 ` Michal Hocko 2020-08-11 15:33 ` Paul E. McKenney 2020-08-11 15:43 ` Thomas Gleixner 2020-08-11 15:56 ` Sebastian Andrzej Siewior 2020-08-11 16:02 ` Paul E. McKenney 2020-08-11 16:19 ` Paul E. McKenney 2020-08-11 19:39 ` Thomas Gleixner 2020-08-11 21:09 ` Paul E. McKenney 2020-08-12 0:13 ` Thomas Gleixner 2020-08-12 4:29 ` Paul E. McKenney 2020-08-12 8:32 ` Thomas Gleixner 2020-08-12 13:30 ` Paul E. McKenney 2020-08-13 7:50 ` Michal Hocko 2020-08-13 9:58 ` Uladzislau Rezki 2020-08-13 11:15 ` Michal Hocko 2020-08-13 13:27 ` Thomas Gleixner 2020-08-13 13:45 ` Michal Hocko 2020-08-13 14:32 ` Matthew Wilcox 2020-08-13 16:14 ` Thomas Gleixner 2020-08-13 16:22 ` Matthew Wilcox 2020-08-13 13:22 ` Thomas Gleixner 2020-08-13 13:33 ` Michal Hocko 2020-08-13 14:34 ` Thomas Gleixner 2020-08-13 14:53 ` Michal Hocko 2020-08-13 15:41 ` Paul E. McKenney 2020-08-13 15:54 ` Michal Hocko 2020-08-13 16:04 ` Paul E. McKenney 2020-08-13 16:13 ` Michal Hocko 2020-08-13 16:29 ` Paul E. McKenney 2020-08-13 17:12 ` Michal Hocko 2020-08-13 17:27 ` Paul E. McKenney 2020-08-13 18:31 ` peterz 2020-08-13 19:13 ` Michal Hocko 2020-08-13 16:20 ` Uladzislau Rezki 2020-08-13 16:36 ` Michal Hocko 2020-08-14 11:54 ` Uladzislau Rezki 2020-08-13 17:09 ` Thomas Gleixner 2020-08-13 17:22 ` Michal Hocko 2020-08-14 7:17 ` Michal Hocko 2020-08-14 12:15 ` Uladzislau Rezki 2020-08-14 12:48 ` Michal Hocko 2020-08-14 13:34 ` Paul E. McKenney 2020-08-14 14:06 ` Michal Hocko 2020-08-14 18:01 ` Paul E. McKenney 2020-08-14 23:14 ` Thomas Gleixner 2020-08-14 23:41 ` Paul E. McKenney 2020-08-15 0:43 ` Thomas Gleixner 2020-08-15 3:01 ` Paul E. McKenney 2020-08-15 8:27 ` Peter Zijlstra 2020-08-15 13:03 ` Paul E. McKenney 2020-08-15 8:42 ` Peter Zijlstra 2020-08-15 14:18 ` Paul E. McKenney 2020-08-15 14:23 ` Paul E. McKenney 2020-08-17 8:47 ` Michal Hocko 2020-08-13 18:26 ` peterz 2020-08-13 18:52 ` Paul E. McKenney 2020-08-13 22:06 ` peterz 2020-08-13 23:23 ` Paul E. McKenney 2020-08-13 23:59 ` Thomas Gleixner 2020-08-14 8:30 ` Peter Zijlstra 2020-08-14 10:23 ` peterz 2020-08-14 15:26 ` Paul E. McKenney 2020-08-14 14:14 ` Paul E. McKenney 2020-08-14 16:11 ` Paul E. McKenney 2020-08-14 17:49 ` Peter Zijlstra 2020-08-14 18:02 ` Paul E. McKenney 2020-08-14 19:33 ` Thomas Gleixner 2020-08-14 20:41 ` Paul E. McKenney 2020-08-14 21:52 ` Peter Zijlstra 2020-08-14 23:27 ` Paul E. McKenney 2020-08-14 23:40 ` Thomas Gleixner 2020-08-16 22:56 ` Uladzislau Rezki 2020-08-17 8:28 ` Michal Hocko 2020-08-17 10:36 ` Uladzislau Rezki 2020-08-17 22:28 ` Paul E. McKenney 2020-08-18 7:43 ` Michal Hocko 2020-08-18 13:53 ` Paul E. McKenney 2020-08-18 14:43 ` Thomas Gleixner 2020-08-18 16:13 ` Paul E. McKenney 2020-08-18 16:55 ` Thomas Gleixner 2020-08-18 17:13 ` Paul E. McKenney 2020-08-18 23:26 ` Thomas Gleixner 2020-08-19 23:07 ` Paul E. McKenney 2020-08-18 15:02 ` Michal Hocko 2020-08-18 15:45 ` Uladzislau Rezki 2020-08-18 16:18 ` Paul E. McKenney 2020-08-14 16:19 ` peterz 2020-08-14 18:15 ` Paul E. McKenney 2020-08-13 13:29 ` Uladzislau Rezki 2020-08-13 13:41 ` Michal Hocko 2020-08-13 14:22 ` Uladzislau Rezki 2020-08-09 20:43 ` [PATCH 2/2] rcu/tree: use " Uladzislau Rezki (Sony)
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