Re: [PATCH tip/core/rcu 14/15] rcu/tree: Allocate a page when caller is preemptible

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Oct 01, 2020 at 11:02:20AM +0200, Michal Hocko wrote:
> On Wed 30-09-20 16:21:54, Paul E. McKenney wrote:
> > On Wed, Sep 30, 2020 at 10:41:39AM +0200, Michal Hocko wrote:
> > > On Tue 29-09-20 18:53:27, Paul E. McKenney wrote:
> [...]
> > > > No argument on it being confusing, and I hope that the added header
> > > > comment helps.  But specifically, can_sleep==true is a promise by the
> > > > caller to be schedulable and not to be holding any lock/mutex/whatever
> > > > that might possibly be acquired by the memory allocator or by anything
> > > > else that the memory allocator might invoke, to your point, including
> > > > for but one example the reclaim logic.
> > > > 
> > > > The only way that can_sleep==true is if this function was invoked due
> > > > to a call to single-argument kvfree_rcu(), which must be schedulable
> > > > because its fallback is to invoke synchronize_rcu().
> > > 
> > > OK. I have to say that it is still not clear to me whether this call
> > > path can be called from the memory reclaim context. If yes then you need
> > > __GFP_NOMEMALLOC as well.
> > 
> > Right now the restriction is that single-argument (AKA can_sleep==true)
> > kvfree_rcu() cannot be invoked from memory reclaim context.
> > 
> > But would adding __GFP_NOMEMALLOC to the can_sleep==true GFP_ flags
> > allow us to remove this restriction?  If so, I will queue a separate
> > patch making this change.  The improved ease of use would be well
> > worth it, if I understand correctly (ha!!!).
> 
> It would be quite daring to claim it will be ok but it will certainly be
> less problematic. Adding the flag will not hurt in any case. As this is
> a shared called that might be called from many contexts I think it will
> be safer to have it there. The justification is that it will prevent
> consumption of memory reserves from MEMALLOC contexts.
> 
> > 
> > > [...]
> > > 
> > > > > What is the point of calling kmalloc  for a PAGE_SIZE object? Wouldn't
> > > > > using the page allocator directly be better?
> > > > 
> > > > Well, you guys gave me considerable heat about abusing internal allocator
> > > > interfaces, and kmalloc() and kfree() seem to be about as non-internal
> > > > as you can get and still be invoking the allocator.  ;-)
> > > 
> > > alloc_pages resp. __get_free_pages is a normal page allocator interface
> > > to use for page size granular allocations. kmalloc is for more fine
> > > grained allocations.
> > 
> > OK, in the short term, both work, but I have queued a separate patch
> > making this change and recording the tradeoffs.  This is not yet a
> > promise to push this patch, but it is a promise not to lose this part
> > of the picture.  Please see below.
> 
> It doesn't matter all that much. Both allocators will work. It is just a
> matter of using optimal tool for the specific purose.
> 
> > You mentioned alloc_pages().  I reverted to __get_free_pages(), but
> > alloc_pages() of course looks nicer.  What are the tradeoffs between
> > __get_free_pages() and alloc_pages()?
> 
> alloc_pages will return struct page but you need a kernel pointer. That
> is what __get_free_pages will give you (or you can call page_address
> directly).
> 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 490b638d7c241ac06cee168ccf8688bb8b872478
> > Author: Paul E. McKenney <paulmck@kernel.org>
> > Date:   Wed Sep 30 16:16:39 2020 -0700
> > 
> >     kvfree_rcu(): Switch from kmalloc/kfree to __get_free_page/free_page.
> >     
> >     The advantages of using kmalloc() and kfree() are a possible small speedup
> >     on CONFIG_SLAB=y systems, avoiding the allocation-side cast, and use of
> >     more-familiar API members.  The advantages of using __get_free_page()
> >     and free_page() are a possible reduction in fragmentation and direct
> >     access to the buddy allocator.
> >     
> >     To help settle the question as to which to use, this commit switches
> >     from kmalloc() and kfree() to __get_free_page() and free_page().
> >     
> >     Suggested-by: Michal Hocko <mhocko@suse.com>
> >     Suggested-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Yes, looks good to me. I am not entirely sure about the fragmentation
> argument. It really depends on the SL.B allocator internals. The same
> applies for the potential speed up. I would be even surprised if the
> SLAB was faster in average considering it has to use the page allocator
> as well. So to me the primary motivation would be "use the right tool
> for the purpose".
> 
As for raised a concern about fragmentation, i mostly was thinking about
that SLAbs where not designed to do an efficient allocations for sizes
which are >= than PAGE_SIZE. But it depends on three different
implementations, actually it also a good argument to switch to the page
allocator. I mean to get rid of such dependency.

Other side is, SLABs, at least SLAB and SLUB use slab-caches and sizes
which they support include up to:

<snip>
kmalloc-8k           420    420   8192    4
kmalloc-4k          1372   1392   4096    8    8 : tunables    0    0
...
<snip>

I would no be surprised that SLAB is faster than using the page allocator
in _some_ sense. If it is principle i can double check. I can explain it
just in having dynamic caching that can grow based on demand, thus there
is no need to go deeper to page allocator if the kmalloc-4k has extra
objects. But the worst case of course will be slower :)

--
Vlad Rezki