On Wed, Sep 30, 2020 at 12:48 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Wed 30-09-20 11:25:17, Joel Fernandes wrote: > > On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote: > > > On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote: > > > > > > > > > > > > > > > > All good points! > > > > > > > > > > > > > > > > On the other hand, duplicating a portion of the allocator functionality > > > > > > > > within RCU increases the amount of reserved memory, and needlessly most > > > > > > > > of the time. > > > > > > > > > > > > > > > > > > > > > > But it's very similar to what mempools are for. > > > > > > > > > > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU > > > > > > to move things forward and it might be not efficient way. As a side > > > > > > effect, maintaining of the bulk arrays in the separate worker thread > > > > > > will introduce other drawbacks: > > > > > > > > > > This is true but it is also true that it is RCU to require this special > > > > > logic and we can expect that we might need to fine tune this logic > > > > > depending on the RCU usage. We definitely do not want to tune the > > > > > generic page allocator for a very specific usecase, do we? > > > > > > > > > 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). > > > > I think documenting is useful. > > > > Could it be more explicit in what the issue is? Something like: > > > > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT > > systems. Therefore, the current low-level allocator implementation does not > > support being called from special contexts that are atomic on RT - such as > > NMI and raw_spin_lock. Due to these constraints and considering calling code > > usually has no control over the PREEMPT_RT configuration, callers of the > > allocator should avoid calling the allocator from these cotnexts even in > > non-RT systems. > > I do not mind documenting RT specific behavior but as mentioned in other > reply, this should likely go via RT tree for now. There is likely more > to clarify about atomicity for PREEMPT_RT. I am sorry, I did not understand what you meant by something missing in Linus Tree. CONFIG_PREEMPT_RT is there. Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing driving this requirement for GFP_ATOMIC right? Or are there non-RT reasons why GFP_ATOMIC allocation cannot be done from NMI/raw_spin_lock ? Thanks, - Joel
