On Wed 30-09-20 15:39:54, Uladzislau Rezki wrote: > On Wed, Sep 30, 2020 at 02:44:13PM +0200, Michal Hocko wrote: > > On Wed 30-09-20 14:35:35, Uladzislau Rezki wrote: > > > On Wed, Sep 30, 2020 at 11:27:32AM +0200, Michal Hocko wrote: > > > > On Tue 29-09-20 18:25:14, Uladzislau Rezki wrote: > > > > > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability > > > > > > > to provide a memory service for contexts which are not allowed to > > > > > > > sleep, RCU is part of them. Both flags used to provide such ability > > > > > > > before but not anymore. > > > > > > > > > > > > > > Do you agree with it? > > > > > > > > > > > > Yes this sucks. But this is something that we likely really want to live > > > > > > with. We have to explicitly _document_ that really atomic contexts in RT > > > > > > cannot use the allocator. From the past discussions we've had this is > > > > > > likely the most reasonable way forward because we do not really want to > > > > > > encourage anybody to do something like that and there should be ways > > > > > > around that. The same is btw. true also for !RT. The allocator is not > > > > > > NMI safe and while we should be able to make it compatible I am not > > > > > > convinced we really want to. > > > > > > > > > > > > Would something like this be helpful wrt documentation? > > > > > > > > > > > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > > > > > > index 67a0774e080b..9fcd47606493 100644 > > > > > > --- a/include/linux/gfp.h > > > > > > +++ b/include/linux/gfp.h > > > > > > @@ -238,7 +238,9 @@ struct vm_area_struct; > > > > > > * %__GFP_FOO flags as necessary. > > > > > > * > > > > > > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > > > > > > - * watermark is applied to allow access to "atomic reserves" > > > > > > + * watermark is applied to allow access to "atomic reserves". > > > > > > + * The current implementation doesn't support NMI and other non-preemptive context > > > > > > + * (e.g. raw_spin_lock). > > > > > > * > > > > > > * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires > > > > > > * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim. > > > > > > > > > > > To me it is clear. But also above conflicting statement: > > > > > > > > > > <snip> > > > > > %GFP_ATOMIC users can not sleep and need the allocation to succeed. A %lower > > > > > <snip> > > > > > > > > > > should be rephrased, IMHO. > > > > > > > > Any suggestions? Or more specifics about which part is conflicting? It > > > > tries to say that there is a higher demand to succeed even though the > > > > context cannot sleep to take active measures to achieve that. So the > > > > only way to achieve that is to break the watermakrs to a certain degree > > > > which is making them more "higher class" than other allocations. > > > > > > > Michal, i had only one concern about it. It says that %GFP_ATOMIC users > > > can not sleep, i.e. callers know that they are in atomic, thus no any > > > sleeping, but the chose they make will force them to sleep. > > > > I am not sure I follow you here. Do you mean they will be forced to > > sleep with PREEMPT_RT? > > > Exactly :) We can make that more specific once RT patchset is merged. As of now this is not the thing in the Linus tree. I believe there will be more to clarify about atomic contexts in the RT tree as it means something else than people are used to think. -- Michal Hocko SUSE Labs
