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

[Michal Hocko]

On Fri 29-01-21 17:35:31, Uladzislau Rezki wrote:
> 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@xxxxxxxxx>
> > > 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@xxxxxxxxx>
> 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@xxxxxxxxx>

Looks good to me. Feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  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

-- 
Michal Hocko
SUSE Labs