On Tue, Mar 31, 2020 at 04:04:33PM +0200, Uladzislau Rezki wrote: > On Tue, Mar 31, 2020 at 09:16:28AM -0400, Joel Fernandes (Google) wrote: > > In kfree_rcu() headless implementation (where the caller need not pass > > an rcu_head, but rather directly pass a pointer to an object), we have > > a fall-back where we allocate a rcu_head wrapper for the caller (not the > > common case). This brings the pattern of needing to allocate some memory > > to free some memory. Currently we use GFP_ATOMIC flag to try harder for > > this allocation, however the GFP_MEMALLOC flag is more tailored to this > > pattern. We need to try harder not only during atomic context, but also > > during non-atomic context anyway. So use the GFP_MEMALLOC flag instead. > > > > Also remove the __GFP_NOWARN flag simply because although we do have a > > synchronize_rcu() fallback for absolutely worst case, we still would > > like to not enter that path and atleast trigger a warning to the user. > > > > Cc: linux-mm@xxxxxxxxx > > Cc: rcu@xxxxxxxxxxxxxxx > > Cc: willy@xxxxxxxxxxxxx > > Cc: peterz@xxxxxxxxxxxxx > > Cc: neilb@xxxxxxxx > > Cc: vbabka@xxxxxxx > > Cc: mgorman@xxxxxxx > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > --- > > > > This patch is based on the (not yet upstream) code in: > > git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree) > > > > It is a follow-up to the posted series: > > https://lore.kernel.org/lkml/20200330023248.164994-1-joel@xxxxxxxxxxxxxxxxx/ > > > > > > kernel/rcu/tree.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > 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); > > > Hello, Joel > > I have some questions regarding improving it, see below them: > > Do you mean __GFP_MEMALLOC? Can that flag be used in atomic context? > Actually we do allocate there under spin lock. Should be combined with > GFP_ATOMIC | __GFP_MEMALLOC? Yes, I mean __GFP_MEMALLOC. Sorry, the patch was just to show the idea and marked as RFC. Good point on the atomic aspect of this path, you are right we cannot sleep. I believe the GFP_NOWAIT I mentioned in my last reply will take care of that? > As for removing __GFP_NOWARN. Actually it is expectable that an > allocation can fail, if so we follow last emergency case. You > can see the trace but what would you do with that information? Yes, the benefit of the trace/warning is that the user can switch to a non-headless API and avoid the synchronize_rcu(), that would help them get faster kfree_rcu() performance instead of having silent slowdowns. It also tells us whether the headless API is worth it in the long run, I think it is worth it because we will likely never hit the synchronize_rcu() failsafe. But if we hit it a lot, at least it wont happen silently. Paul was concerned about following scenario with hitting synchronize_rcu(): 1. Consider a system under memory pressure. 2. Consider some other subsystem X depending on another system Y which uses kfree_rcu(). If Y doesn't complete the operation in time, X accumulates more memory. 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down. This causes X to further allocate memory, further causing a chain reaction. Paul, please correct me if I'm wrong. thanks, - Joel
