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

Uladzislau Rezki
2020-10-01T20:03:43Z

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