Re: [PATCH] tracing/ring_buffer: Try harder to allocate

[Michal Hocko]

On Mon 10-07-17 23:05:00, Joel Fernandes wrote:
> 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

Yes this is the correct usage of the new flag.

> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Hao Lee <haolee.swjtu@xxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Tim Murray <timmurray@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

I do not think stable tag is appropriate. The new flag hasn't been
merged yet and it is not a stable material.

> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>

Feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  kernel/trace/ring_buffer.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 4ae268e687fe..529cc50d7243 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/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);
> -- 
> 2.13.2.725.g09c95d1e9-goog

-- 
Michal Hocko
SUSE Labs