Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Jan 29, 2021 at 09:56:29AM +0100, Michal Hocko wrote:
> On Thu 28-01-21 19:02:37, Uladzislau Rezki wrote:
> [...]
> > >From 0bdb8ca1ae62088790e0a452c4acec3821e06989 Mon Sep 17 00:00:00 2001
> > From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
> > Date: Wed, 20 Jan 2021 17:21:46 +0100
> > Subject: [PATCH v2 1/1] kvfree_rcu: Directly allocate page for single-argument
> >  case
> > 
> > Single-argument kvfree_rcu() must be invoked from sleepable contexts,
> > so we can directly allocate pages.  Furthermmore, the fallback in case
> > of page-allocation failure is the high-latency synchronize_rcu(), so it
> > makes sense to do these page allocations from the fastpath, and even to
> > permit limited sleeping within the allocator.
> > 
> > This commit therefore allocates if needed on the fastpath using
> > GFP_KERNEL|__GFP_NORETRY.
> 
> Yes, __GFP_NORETRY as a lightweight allocation mode should be fine. It
> is more robust than __GFP_NOWAIT on memory usage spikes.  The caller is
> prepared to handle the failure which is likely much less disruptive than
> OOM or potentially heavy reclaim __GFP_RETRY_MAYFAIL.
> 
> I cannot give you ack as I am not familiar with the code but this makes
> sense to me.
> 
No problem, i can separate it. We can have a patch on top of what we have so
far. The patch only modifies the gfp_mask passed to __get_free_pages():

From ec2feaa9b7f55f73b3b17e9ac372151c1aab5ae0 Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Date: Fri, 29 Jan 2021 17:16:03 +0100
Subject: [PATCH 1/1] kvfree_rcu: replace __GFP_RETRY_MAYFAIL by __GFP_NORETRY

__GFP_RETRY_MAYFAIL is a bit heavy from reclaim process of view,
therefore a time consuming. That is not optional and there is
no need in doing it so hard, because we have a fallback path.

__GFP_NORETRY in its turn can perform some light-weight reclaim
and it rather fails under high memory pressure or low memory
condition.

In general there are four simple criterias we we would like to
achieve:
    a) minimize a fallback hitting;
    b) avoid of OOM invoking;
    c) do a light-wait page request;
    d) avoid of dipping into the emergency reserves.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 70ddc339e0b7..1e862120db9e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3489,8 +3489,20 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 		bnode = get_cached_bnode(*krcp);
 		if (!bnode && can_alloc) {
 			krc_this_cpu_unlock(*krcp, *flags);
+
+			// __GFP_NORETRY - allows a light-weight direct reclaim
+			// what is OK from minimizing of fallback hitting point of
+			// view. Apart of that it forbids any OOM invoking what is
+			// also beneficial since we are about to release memory soon.
+			//
+			// __GFP_NOMEMALLOC - prevents from consuming of all the
+			// memory reserves. Please note we have a fallback path.
+			//
+			// __GFP_NOWARN - it is supposed that an allocation can
+			// be failed under low memory or high memory pressure
+			// scenarios.
 			bnode = (struct kvfree_rcu_bulk_data *)
-				__get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
+				__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
 			*krcp = krc_this_cpu_lock(flags);
 		}
 
-- 
2.20.1

--
Vlad Rezki