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SubjectRe: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern
On Wed, Apr 01, 2020 at 09:19:49AM +1100, NeilBrown wrote:
> On Tue, Mar 31 2020, Joel Fernandes wrote:
>
> > On Tue, Mar 31, 2020 at 05:34:50PM +0200, Michal Hocko wrote:
> >> On Tue 31-03-20 10:58:06, Joel Fernandes wrote:
> >> [...]
> >> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> > > index 4be763355c9fb..965deefffdd58 100644
> >> > > --- a/kernel/rcu/tree.c
> >> > > +++ b/kernel/rcu/tree.c
> >> > > @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
> >> > >
> >> > > if (!ptr)
> >> > > ptr = kmalloc(sizeof(unsigned long *) +
> >> > > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
> >> > > + sizeof(struct rcu_head), GFP_MEMALLOC);
> >> >
> >> > Just to add, the main requirements here are:
> >> > 1. Allocation should be bounded in time.
> >> > 2. Allocation should try hard (possibly tapping into reserves)
> >> > 3. Sleeping is Ok but should not affect the time bound.
> >>
> >>
> >> __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to
> >> memory reserves regarless of the sleeping status.
> >>
> >> Using __GFP_MEMALLOC is quite dangerous because it can deplete _all_ the
> >> memory. What does prevent the above code path to do that?
> >
> > Can you suggest what prevents other users of GFP_MEMALLOC from doing that
> > also? That's the whole point of having a reserve, in normal usage no one will
> > use it, but some times you need to use it. Keep in mind this is not a common
> > case in this code here, this is triggered only if earlier allocation attempts
> > failed. Only *then* we try with GFP_MEMALLOC with promises to free additional
> > memory soon.
>
> I think that "soon" is the key point. Users of __GFP_MEMALLOC certainly
> must be working to free other memory, that other memory needs to be freed
> "soon". In particular - sooner than all the reserve is exhausted. This
> can require rate-limiting. If one allocation can result in one page
> being freed, that is good and it is probably OK to have 1000 allocations
> resulting in 1000 pages being freed soon. But 10 million allocation to
> gain 10 million pages is not such a good thing and shouldn't be needed.
> Once those first 1000 pages have been freed, you won't need
> __GFP_MEMALLOC allocations any more, and you must be prepare to wait for
> them.
>
> So where does the rate-limiting happen in your proposal? A GP can be
> multiple milliseconds, which is time for lots of memory to be allocated
> and for rcu-free queues to grow quite large.
>
> You mention a possible fall-back of calling synchronize_rcu(). I think
> that needs to be a fallback that happens well before __GFP_MEMALLOC is
> exhausted. You need to choose some maximum amount that you will
> allocate, then use synchronize_rcu() (or probably the _expedited
> version) after that. The pool of reserves are certainly there for you
> to use, but not for you to exhaust.
>
> If you have your own rate-limiting, then I think __GFP_MEMALLOC is
> probably OK, and also you *don't* want the memalloc to wait. If memory
> cannot be allocated immediately, you need to use your own fallback.

Thanks a lot for explaining in detail, the RFC patch has served its purpose
well ;-)

On discussing with RCU comrades, we agreed to not use GFP_MEMALLOC. But
instead pre-allocate a cache (we do have a cache but it is not yet
pre-allocated, just allocated on demand).

About the rate limiting, we would fallback to synchronize_rcu() instead of
sleeping in case of trobule. However I would like to add a warning if we ever
hit the troublesome path mainly because that means we depleted the
pre-allocated cache and perhaps the user should switch to adding an rcu_head
in their structure to reduce latency. I'm adding that warning to my tree:

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4be763355c9fb..6172e6296dd7d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -110,6 +110,10 @@ module_param(rcu_fanout_exact, bool, 0444);
static int rcu_fanout_leaf = RCU_FANOUT_LEAF;
module_param(rcu_fanout_leaf, int, 0444);
int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
+/* Silence the kvfree_rcu() complaint (warning) that it blocks */
+int rcu_kfree_nowarn;
+module_param(rcu_kfree_nowarn, int, 0444);
+
/* Number of rcu_nodes at specified level. */
int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
@@ -3266,6 +3270,12 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
* state.
*/
if (!success) {
+ /*
+ * Please embed an rcu_head and pass it along if you hit this
+ * warning. Doing so would avoid long kfree_rcu() latencies.
+ */
+ if (!rcu_kfree_nowarn)
+ WARN_ON_ONCE(1);
debug_rcu_head_unqueue(ptr);
synchronize_rcu();
kvfree(ptr);
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