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)" <[email protected]> > > 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)" <[email protected]> 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) <[email protected]> --- 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
