Re: [PATCH 15/16] rcu/tree: Allocate a page when caller is preemptible

[Uladzislau Rezki]

Hello, Joel.

> 
> On Thu, Oct 29, 2020 at 05:50:18PM +0100, Uladzislau Rezki (Sony) wrote:
> > Given that CONFIG_PREEMPT_COUNT is unconditionally enabled by the
> > earlier commits in this series, the preemptible() macro now properly
> > detects preempt-disable code regions even in kernels built with
> > CONFIG_PREEMPT_NONE.
> > 
> > This commit therefore uses preemptible() to determine whether allocation
> > is possible at all for double-argument kvfree_rcu().  If !preemptible(),
> > then allocation is not possible, and kvfree_rcu() falls back to using
> > the less cache-friendly rcu_head approach.  Even when preemptible(),
> > the caller might be involved in reclaim, so the GFP_ flags used by
> > double-argument kvfree_rcu() must avoid invoking reclaim processing.
> > 
> > Note that 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@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >  kernel/rcu/tree.c | 48 ++++++++++++++++++++++++++++++-----------------
> >  1 file changed, 31 insertions(+), 17 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f2da2a1cc716..3f9b016a44dc 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3406,37 +3406,55 @@ 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_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 if ptr was
> > +// successfully recorded, else the caller must use a fallback.
> >  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_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;
> >  	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. */
> 
> Maybe convert this comment also to //... like the new ones you added (and the
> ones below).
> 
No, problem. I can convert it.

> > -	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_page) {
> 
> I think you can directly put preemptible() here with a comment saying
> allocate only if preemptible and get rid of can_alloc_page.
> 
Not really. We check preemtable() before acquiring the internal lock,
otherwise it will always return "false". Thus, it is checked on the
entry in the beginning.

> > +			krc_this_cpu_unlock(*krcp, *flags);
> > +			bnode = (struct kvfree_rcu_bulk_data *)
> > +				__get_free_page(gfp);
> > +			*krcp = krc_this_cpu_lock(flags);
> > +		}
> > +
> 
> I think the "Switch to emergency path" comment should stay here before the
> if().
> 
Right. We need to keep it. Will take it back.

--
Vlad Rezki