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
next prev parent 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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