tracing/ring_buffer: Try harder to allocate

ftrace can fail to allocate per-CPU ring buffer on systems with a large
number of CPUs coupled while large amounts of cache happening in the
page cache. Currently the ring buffer allocation doesn't retry in the VM
implementation even if direct-reclaim made some progress but still
wasn't able to find a free page. On retrying I see that the allocations
almost always succeed. The retry doesn't happen because __GFP_NORETRY is
used in the tracer to prevent the case where we might OOM, however if we
drop __GFP_NORETRY, we risk destabilizing the system if OOM killer is
triggered. To prevent this situation, use the __GFP_RETRY_MAYFAIL flag
introduced recently [1].

Tested the following still succeeds without destabilizing a system with
1GB memory.
echo 300000 > /sys/kernel/debug/tracing/buffer_size_kb

[1] https://marc.info/?l=linux-mm&m=149820805124906&w=2

Link: http://lkml.kernel.org/r/20170713021416.8897-1-joelaf@google.com

Cc: Tim Murray <timmurray@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -1136,12 +1136,12 @@ static int __rb_allocate_pages(long nr_pages, struct list_head *pages, int cpu)
 	for (i = 0; i < nr_pages; i++) {
 		struct page *page;
 		/*
-		 * __GFP_NORETRY flag makes sure that the allocation fails
-		 * gracefully without invoking oom-killer and the system is
-		 * not destabilized.
+		 * __GFP_RETRY_MAYFAIL flag makes sure that the allocation fails
+		 * gracefully without invoking oom-killer and the system is not
+		 * destabilized.
 		 */
 		bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
-				    GFP_KERNEL | __GFP_NORETRY,
+				    GFP_KERNEL | __GFP_RETRY_MAYFAIL,
 				    cpu_to_node(cpu));
 		if (!bpage)
 			goto free_pages;
@@ -1149,7 +1149,7 @@ static int __rb_allocate_pages(long nr_pages, struct list_head *pages, int cpu)
 		list_add(&bpage->list, pages);
 
 		page = alloc_pages_node(cpu_to_node(cpu),
-					GFP_KERNEL | __GFP_NORETRY, 0);
+					GFP_KERNEL | __GFP_RETRY_MAYFAIL, 0);
 		if (!page)
 			goto free_pages;
 		bpage->page = page_address(page);