Messages in this thread Patch in this message | | | Date | Wed, 30 Sep 2020 16:21:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 14/15] rcu/tree: Allocate a page when caller is preemptible |
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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: > > On Tue, Sep 29, 2020 at 02:07:56PM +0200, Michal Hocko wrote: > > > On Mon 28-09-20 16:31:01, paulmck@kernel.org wrote: > > > [...] > > > > Apologies for the delay, but today has not been boring. > > > > > > This commit therefore uses preemptible() to determine whether allocation > > > > is possible at all for double-argument kvfree_rcu(). > > > > > > This deserves a comment. Because GFP_ATOMIC is possible for many > > > !preemptible() contexts. It is the raw_spin_lock, NMIs and likely few > > > others that are a problem. You are taking a conservative approach which > > > is fine but it would be good to articulate that explicitly. > > > > Good point, and so I have added the following as a header comment to > > the add_ptr_to_bulk_krc_lock() function: > > > > // Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock() > > // state specified by flags. If can_sleep is true, the caller must > > // be schedulable and not be holding any locks or mutexes that might be > > // acquired by the memory allocator or anything that it might invoke. > > // If !can_sleep, then if !preemptible() no allocation will be undertaken, > > // otherwise the allocation will use GFP_ATOMIC to avoid the remainder of > > // the aforementioned deadlock possibilities. Returns true iff ptr was > > // successfully recorded, else the caller must use a fallback. > > OK, not trivial to follow but at least verbose enough to understand the > intention after some mulling. Definitely an improvement, thanks!
Glad it helped! With some luck, perhaps it will improve with time...
> [...] > > > > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) > > > > +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, > > > > + unsigned long *flags, void *ptr, bool can_sleep) > > > > { > > > > struct kvfree_rcu_bulk_data *bnode; > > > > + bool can_alloc_page = preemptible(); > > > > + gfp_t gfp = (can_sleep ? GFP_KERNEL | __GFP_RETRY_MAYFAIL : GFP_ATOMIC) | __GFP_NOWARN; > > > > > > This is quite confusing IMHO. At least without a further explanation. > > > can_sleep is not as much about sleeping as it is about the reclaim > > > recursion AFAIU your changelog, right? > > > > 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!!!).
> [...] > > > > 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.
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()?
Thanx, Paul
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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>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2886e81..242f0f0 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3225,7 +3225,8 @@ static void kfree_rcu_work(struct work_struct *work) bkvhead[i] = NULL; krc_this_cpu_unlock(krcp, flags); - kfree(bkvhead[i]); + if (bkvhead[i]) + free_page((unsigned long)bkvhead[i]); cond_resched_tasks_rcu_qs(); } @@ -3378,7 +3379,7 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, bnode = get_cached_bnode(*krcp); if (!bnode && can_alloc_page) { krc_this_cpu_unlock(*krcp, *flags); - bnode = kmalloc(PAGE_SIZE, gfp); + bnode = (struct kvfree_rcu_bulk_data *)__get_free_page(gfp); *krcp = krc_this_cpu_lock(flags); }
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