Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Jan 20, 2021 at 08:57:57PM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-01-20 17:21:46 [+0100], Uladzislau Rezki (Sony) wrote:
> > For a single argument we can directly request a page from a caller
> > context when a "carry page block" is run out of free spots. Instead
> > of hitting a slow path we can request an extra page by demand and
> > proceed with a fast path.
> > 
> > A single-argument kvfree_rcu() must be invoked in sleepable contexts,
> > and that its fallback is the relatively high latency synchronize_rcu().
> > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL
> > to allow limited sleeping within the memory allocator.
> > 
> > [ paulmck: Add add_ptr_to_bulk_krc_lock header comment per Michal Hocko. ]
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  kernel/rcu/tree.c | 42 ++++++++++++++++++++++++++----------------
> >  1 file changed, 26 insertions(+), 16 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index e04e336bee42..2014fb22644d 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3465,37 +3465,50 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
> >  	}
> >  }
> >  
> > +// Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
> > +// state specified by flags.  If can_alloc 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.
> > +// Returns true if ptr was successfully recorded, else the caller must
> > +// use a fallback.
> 
> The whole RCU department is getting swamped by the // comments. Can't we
> have proper kernel doc and /* */ style comments like the remaining part
> of the kernel?
> 
> >  static inline bool
> > -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_alloc)
> >  {
> >  	struct kvfree_rcu_bulk_data *bnode;
> >  	int idx;
> >  
> > -	if (unlikely(!krcp->initialized))
> > +	*krcp = krc_this_cpu_lock(flags);
> > +	if (unlikely(!(*krcp)->initialized))
> >  		return false;
> >  
> > -	lockdep_assert_held(&krcp->lock);
> >  	idx = !!is_vmalloc_addr(ptr);
> >  
> >  	/* Check if a new block is required. */
> > -	if (!krcp->bkvhead[idx] ||
> > -			krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > -		bnode = get_cached_bnode(krcp);
> > -		/* Switch to emergency path. */
> > +	if (!(*krcp)->bkvhead[idx] ||
> > +			(*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > +		bnode = get_cached_bnode(*krcp);
> > +		if (!bnode && can_alloc) {
> > +			krc_this_cpu_unlock(*krcp, *flags);
> > +			bnode = (struct kvfree_rcu_bulk_data *)
> 
> There is no need for this cast.
> 
__get_free_page() returns "unsigned long" whereas a bnode is a pointer
to kvfree_rcu_bulk_data struct, without a casting the compiler will
emit a warning.

> > +				__get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
> > +			*krcp = krc_this_cpu_lock(flags);
> 
> so if bnode is NULL you could retry get_cached_bnode() since it might
> have been filled (given preemption or CPU migration changed something).
> Judging from patch #3 you think that a CPU migration is a bad thing. But
> why?
> 
I see your point. Indeed we can retry but honestly i do not see that it
makes a lot of sense. I prefer to keep the logic as simple as it can be.
If we are run out of free pages(low memory condition), there is a fallback
mechanism for such purpose, i.e it implies that a slow path can be hit.

>>
>> You think that a CPU migration is a bad thing. But why?
>>
It is not a bad thing. But if it happens we might queue a new bnode
to a drain list of another CPU where a previous element of a new
bnode may be just underutilized. So that is why i use migrate_disable()/enable()
to prevent it.

If there are some hidden issues with migrate_disable()/enable() or you
think it is a bad idea to use it, it would be appreciated if you could
describe your view in more detail.

Thanks for the comments!

--
Vlad Rezki