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From: Uladzislau Rezki <urezki@gmail.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	paulmck@kernel.org, Uladzislau Rezki <urezki@gmail.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>,
	Matthew Wilcox <willy@infradead.org>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Joel Fernandes <joel@joelfernandes.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
Date: Thu, 13 Aug 2020 11:58:40 +0200	[thread overview]
Message-ID: <20200813095840.GA25268@pc636> (raw)
In-Reply-To: <20200813075027.GD9477@dhcp22.suse.cz>

On Thu, Aug 13, 2020 at 09:50:27AM +0200, Michal Hocko wrote:
> On Wed 12-08-20 02:13:25, Thomas Gleixner wrote:
> [...]
> > I can understand your rationale and what you are trying to solve. So, if
> > we can actually have a distinct GFP variant:
> > 
> >   GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY
> 
> Even if we cannot make the zone->lock raw I would prefer to not
> introduce a new gfp flag. Well we can do an alias for easier grepping
> #define GFP_RT_SAFE	0
> 
> that would imply nowait semantic and would exclude waking up kswapd as
> well. If we can make wake up safe under RT then the alias would reflect
> that without any code changes.
> 
> The second, and the more important part, would be to bail out anytime
> the page allocator is to take a lock which is not allowed in the current
> RT context. Something like 
> 	
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..3ef3ac82d110 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -237,6 +237,9 @@ struct vm_area_struct;
>   * that subsystems start with one of these combinations and then set/clear
>   * %__GFP_FOO flags as necessary.
>   *
> + * %GFP_RT_SAFE users can not sleep and they are running under RT atomic context
> + * e.g. under raw_spin_lock. Failure of an allocation is to be expected.
> + *
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
>   * watermark is applied to allow access to "atomic reserves"
>   *
> @@ -293,6 +296,7 @@ struct vm_area_struct;
>   * version does not attempt reclaim/compaction at all and is by default used
>   * in page fault path, while the non-light is used by khugepaged.
>   */
> +#define GFP_RT_SAFE	0
>  #define GFP_ATOMIC	(__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
>  #define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)
>  #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e028b87ce294..268ae872cc2a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2824,6 +2824,13 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>  {
>  	int i, alloced = 0;
>  
> +	/*
> +	 * Hard atomic contexts are not supported by the allocator for
> +	 * anything but pcp requests
> +	 */
> +	if (!preemtable())
> +		return 0;
> +
>  	spin_lock(&zone->lock);
>  	for (i = 0; i < count; ++i) {
>  		struct page *page = __rmqueue(zone, order, migratetype,
> @@ -3371,6 +3378,13 @@ struct page *rmqueue(struct zone *preferred_zone,
>  		goto out;
>  	}
>  
> +	/*
> +	 * Hard atomic contexts are not supported by the allocator for high
> +	 * order requests
> +	 */
> +	if (WARN_ON_ONCE(!preemtable()))
> +		reurn NULL;
> +
>  	/*
>  	 * We most definitely don't want callers attempting to
>  	 * allocate greater than order-1 page units with __GFP_NOFAIL.
> @@ -4523,6 +4537,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
>  		gfp_mask &= ~__GFP_ATOMIC;
>  
> +	/* Hard atomic contexts support is very limited to the fast path */
> +	if (!preemtable()) {
> +		WARN_ON_ONCE(gfp_mask != GFP_RT_SAFE);
> +		return NULL;
> +	}
> +
>  retry_cpuset:
>  	compaction_retries = 0;
>  	no_progress_loops = 0;
> 
> What do you think?
>  
> > which is easy to grep for then having the page allocator go down to the
> > point where zone lock gets involved is not the end of the world for
> > RT in theory - unless that damned reality tells otherwise. :)
> > 
> > The page allocator allocations should also have a limit on the number of
> > pages and eventually also page order (need to stare at the code or let
> > Michal educate me that the order does not matter).
> 
> In practice anything but order 0 is out of question because we need
> zone->lock for that currently. Maybe we can introduce pcp lists for
> higher orders in the future - I have a vague recollection Mel was
> playing with that some time ago.
> 
> > To make it consistent the same GFP_ variant should allow the slab
> > allocator go to the point where the slab cache is exhausted.
> > 
> > Having a distinct and clearly defined GFP_ variant is really key to
> > chase down offenders and to make reviewers double check upfront why this
> > is absolutely required.
> 
> Having a high level and recognizable gfp mask is OK but I would really
> like not to introduce a dedicated flag. The page allocator should be
> able to recognize the context which cannot be handled. 
>
Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT
is enabled, something like below:

if (IS_ENABLED_RT && preemptebale())

Also i have a question about pcp-lists. Can we introduce and use all allowed 
MIGRATE_PCPTYPES? If called with GFP_RT_SAFE? If not please elaborate.
According to my tests it really helps when it comes to: succeed(return the page) or NULL.
Because on of the list of below types:
 MIGRATE_UNMOVABLE,
 MIGRATE_MOVABLE,
 MIGRATE_RECLAIMABLE,

can have a page making allocation succeed.

Thanks!

--
Vlad Rezki

  reply	other threads:[~2020-08-13  9:58 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
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 [this message]
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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